Make package PEP 561 compatible
Pandas has growing number of type annotations but these are not properly advertised to type checking tools. According to [PEP 561](https://www.python.org/dev/peps/pep-0561/) packages providing annotations should contain `py.typed` files in corresponding packages.

`py.typed` is applied recursively, if placed in the root, and can accept missing annotations, if contains `partial\n`.

This PR adds such file and includes it in `package_data`.

- [x] closes #28142
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Cool thanks for submitting this - makes sense to do in the long run.

Any chance you have personal experience doing this on other projects? My main concern would be the maturity of annotations; we have them but they aren't necessarily comprehensive yet. 

That may or may not be a blocker to doing this just curious if you or anyone you know has experience to know the pros and cons of putting these out there @simonjayhawkins might have thoughts as well > we have them but they aren't necessarily comprehensive yet.

I think that a bigger problem is that Pandas doesn't type check at the moment.  Right now (bee17d5f4c99e6d1e451cf9a7b6a1780aa25988d) there are 55 errors. Some are easy fix (for example #28843, or some other [minor fixes I am working on](https://github.com/zero323/pandas/tree/typing-blockers)), other might be much harder to tackle (there is a bunch of inheritance issues, which are real pain). 

Nonetheless I'd say that, conditioning on fixing or type-ignoring aforementioned problem exposing (even partial) annotations is useful (please bear in mind that I am not objective, as lack of Pandas annotations is blocker for my own work), as long as individual modules (not necessarily all) provide complete, even if mostly dynamic annotations. > I think that a bigger problem is that Pandas doesn't type check at the moment. Right now ([bee17d5](https://github.com/pandas-dev/pandas/commit/bee17d5f4c99e6d1e451cf9a7b6a1780aa25988d)) there are 55 errors

We do type check with mypy as part of our CI - are those failures accounting for what's in our `setup.cfg` to ignore some third party libs without annotations? > > I think that a bigger problem is that Pandas doesn't type check at the moment. Right now ([bee17d5](https://github.com/pandas-dev/pandas/commit/bee17d5f4c99e6d1e451cf9a7b6a1780aa25988d)) there are 55 errors
> 
> We do type check with mypy as part of our CI - are those failures accounting for what's in our `setup.cfg` to ignore some third party libs without annotations?

I am running Mypy directly with `--ignore-missing-imports --no-implicit-optional` (I believe these are the ones that you use in CI pipeline).

```
$ git rev-parse HEAD                                                    
39602e7d7c5b663696f5a6eca9e66e65483bc868
$ mypy -V    
mypy 0.740+dev.beec11a28fd835acb1e4ee8b2bf14d969c4e5922
```

The result is 

```
pandas/io/formats/css.py:232: error: Argument 1 to "_side_expander" has incompatible type "str"; expected "CSSResolver"
pandas/io/formats/css.py:233: error: Argument 1 to "_side_expander" has incompatible type "str"; expected "CSSResolver"
pandas/io/formats/css.py:234: error: Argument 1 to "_side_expander" has incompatible type "str"; expected "CSSResolver"
pandas/io/formats/css.py:235: error: Argument 1 to "_side_expander" has incompatible type "str"; expected "CSSResolver"
pandas/io/formats/css.py:236: error: Argument 1 to "_side_expander" has incompatible type "str"; expected "CSSResolver"
pandas/core/dtypes/dtypes.py:220: error: Incompatible types in assignment (expression has type "dtype", base class "PandasExtensionDtype" defined the type as "None")
pandas/core/dtypes/dtypes.py:657: error: Incompatible types in assignment (expression has type "dtype", base class "PandasExtensionDtype" defined the type as "None")
pandas/core/dtypes/dtypes.py:818: error: Incompatible types in assignment (expression has type "dtype", base class "PandasExtensionDtype" defined the type as "None")
pandas/core/dtypes/dtypes.py:977: error: Incompatible types in assignment (expression has type "dtype", base class "PandasExtensionDtype" defined the type as "None")
pandas/core/dtypes/common.py:144: error: Function "numpy.array" is not valid as a type
pandas/core/dtypes/common.py:144: note: Perhaps you need "Callable[...]" or a callback protocol?
pandas/io/common.py:576: error: Incompatible types in assignment (expression has type "str", variable has type "bytes")
pandas/io/common.py:583: error: Incompatible return value type (got "bytes", expected "str")
pandas/core/nanops.py:312: error: Incompatible types in assignment (expression has type "Type[int64]", variable has type "dtype")
pandas/core/nanops.py:314: error: Incompatible types in assignment (expression has type "Type[float64]", variable has type "dtype")
pandas/core/nanops.py:666: error: Incompatible types in assignment (expression has type "Union[bool, Any]", variable has type "ndarray")
pandas/core/nanops.py:1121: error: Incompatible types in assignment (expression has type "int", variable has type "ndarray")
pandas/core/nanops.py:1124: error: Incompatible types in assignment (expression has type "int", variable has type "ndarray")
pandas/core/arrays/sparse/dtype.py:67: error: Incompatible default for argument "dtype" (default has type "Type[float64]", argument has type "Union[str, dtype, ExtensionDtype]")
pandas/core/construction.py:267: error: Incompatible types in assignment (expression has type "Union[Type[ExtensionDtype], str]", variable has type "Union[str, dtype, ExtensionDtype, None]")
pandas/core/ops/dispatch.py:130: error: Argument 1 to "array" has incompatible type "ndarray"; expected "Sequence[object]"
pandas/core/ops/__init__.py:153: error: "datetime64" has no attribute "astype"
pandas/core/arrays/datetimelike.py:453: error: Argument 1 of "__setitem__" is incompatible with supertype "ExtensionArray"; supertype defines the argument type as "Union[int, ndarray]"
pandas/core/arrays/datetimes.py:604: error: Return type "Union[dtype, DatetimeTZDtype]" of "dtype" incompatible with return type "ExtensionDtype" in supertype "ExtensionArray"
pandas/core/indexes/frozen.py:71: error: Incompatible types in assignment (expression has type "Callable[[FrozenList, Any], Any]", base class "list" defined the type as "Callable[[List[Any], List[Any]], List[Any]]")
pandas/core/indexes/frozen.py:113: error: Incompatible types in assignment (expression has type "Callable[[FrozenList, VarArg(Any), KwArg(Any)], Any]", base class "list" defined the type as overloaded function)
pandas/core/indexes/frozen.py:113: error: Incompatible types in assignment (expression has type "Callable[[FrozenList, VarArg(Any), KwArg(Any)], Any]", base class "list" defined the type as "Callable[[List[Any], Union[int, slice]], None]")
pandas/core/indexes/frozen.py:114: error: Incompatible types in assignment (expression has type "Callable[[FrozenList, VarArg(Any), KwArg(Any)], Any]", base class "list" defined the type as "Callable[[List[Any], int], Any]")
pandas/core/indexes/frozen.py:114: error: Incompatible types in assignment (expression has type "Callable[[FrozenList, VarArg(Any), KwArg(Any)], Any]", base class "list" defined the type as "Callable[[List[Any], Any], None]")
pandas/core/indexes/frozen.py:114: error: Incompatible types in assignment (expression has type "Callable[[FrozenList, VarArg(Any), KwArg(Any)], Any]", base class "list" defined the type as "Callable[[List[Any], Iterable[Any]], None]")
pandas/core/indexes/frozen.py:114: error: Incompatible types in assignment (expression has type "Callable[[FrozenList, VarArg(Any), KwArg(Any)], Any]", base class "list" defined the type as "Callable[[List[Any], DefaultNamedArg(Optional[Callable[[Any], Any]], 'key'), DefaultNamedArg(bool, 'reverse')], None]")
pandas/core/indexes/frozen.py:138: error: Incompatible types in assignment (expression has type "Callable[[FrozenNDArray, VarArg(Any), KwArg(Any)], Any]", base class "ndarray" defined the type as overloaded function)
pandas/core/arrays/categorical.py:521: error: Incompatible return value type (got "Categorical", expected "ndarray")
pandas/core/arrays/categorical.py:522: error: Incompatible return value type (got "Categorical", expected "ndarray")
pandas/core/arrays/categorical.py:528: error: Incompatible return value type (got "ndarray", expected "ExtensionArray")
pandas/core/arrays/categorical.py:528: error: Argument "dtype" to "array" has incompatible type "Union[str, dtype, ExtensionDtype]"; expected "Union[dtype, None, type, str, Tuple[Any, int], Tuple[Any, Union[int, Sequence[int]]], List[Union[Tuple[Union[str, Tuple[str, str]], Any], Tuple[Union[str, Tuple[str, str]], Any, Union[int, Sequence[int]]]]], Dict[str, Union[Sequence[str], Sequence[Any], Sequence[int], Sequence[Union[bytes, str, None]], int]], Dict[str, Tuple[Any, int]], Tuple[Any, Any]]"
pandas/core/indexes/interval.py:1367: error: Argument 1 to "_setop" has incompatible type "str"; expected "IntervalIndex"
pandas/core/indexes/interval.py:1368: error: Argument 1 to "_setop" has incompatible type "str"; expected "IntervalIndex"
pandas/core/indexes/interval.py:1369: error: Argument 1 to "_setop" has incompatible type "str"; expected "IntervalIndex"
pandas/io/formats/format.py:871: error: Argument 2 to "_binify" has incompatible type "Optional[int]"; expected "Union[int32, int]"
pandas/io/formats/format.py:1523: error: Unsupported operand types for >= ("List[Union[int, float]]" and "int")
pandas/io/formats/format.py:1523: error: Unsupported operand types for >= ("List[float]" and "int")
pandas/io/formats/format.py:1523: error: Unsupported operand types for >= ("List[Union[str, float]]" and "int")
pandas/io/formats/format.py:1523: note: Left operand is of type "Union[ndarray, List[Union[int, float]], List[float], List[Union[str, float]]]"
pandas/io/formats/format.py:1524: error: Unsupported operand types for <= ("List[Union[int, float]]" and "int")
pandas/io/formats/format.py:1524: error: Unsupported operand types for <= ("List[float]" and "int")
pandas/io/formats/format.py:1524: error: Unsupported operand types for <= ("List[Union[str, float]]" and "int")
pandas/io/formats/format.py:1524: note: Left operand is of type "Union[ndarray, List[Union[int, float]], List[float], List[Union[str, float]]]"
pandas/io/formats/format.py:1529: error: Item "List[Union[int, float]]" of "Union[Any, List[Union[int, float]], List[Union[str, float]]]" has no attribute "astype"
pandas/io/formats/format.py:1529: error: Item "List[Union[str, float]]" of "Union[Any, List[Union[int, float]], List[Union[str, float]]]" has no attribute "astype"
pandas/io/formats/format.py:1532: error: Item "List[Union[int, float]]" of "Union[Any, List[Union[int, float]], List[Union[str, float]]]" has no attribute "astype"
pandas/io/formats/format.py:1532: error: Item "List[Union[str, float]]" of "Union[Any, List[Union[int, float]], List[Union[str, float]]]" has no attribute "astype"
pandas/core/series.py:803: error: "Type[ndarray]" has no attribute "__array_ufunc__"
pandas/core/frame.py:472: error: Argument "dtype" to "array" has incompatible type "Union[str, dtype, ExtensionDtype, None]"; expected "Union[dtype, None, type, str, Tuple[Any, int], Tuple[Any, Union[int, Sequence[int]]], List[Union[Tuple[Union[str, Tuple[str, str]], Any], Tuple[Union[str, Tuple[str, str]], Any, Union[int, Sequence[int]]]]], Dict[str, Union[Sequence[str], Sequence[Any], Sequence[int], Sequence[Union[bytes, str, None]], int]], Dict[str, Tuple[Any, int]], Tuple[Any, Any]]"
pandas/core/frame.py:482: error: Argument 1 to "len" has incompatible type "Iterable[Any]"; expected "Sized"
pandas/core/groupby/groupby.py:1889: error: Incompatible types in assignment (expression has type "str", variable has type "Optional[Type[int64]]")
pandas/compat/pickle_compat.py:71: error: Incompatible return type for "__new__" (returns "Series", but must return a subtype of "_LoadSparseSeries")
pandas/compat/pickle_compat.py:85: error: Incompatible return type for "__new__" (returns "DataFrame", but must return a subtype of "_LoadSparseFrame")
pandas/io/excel/_odfreader.py:63: error: Cannot find replacement for named format specifier "name"
pandas/io/excel/_odfreader.py:63: error: Not all arguments converted during string formatting
pandas/core/window/rolling.py:243: error: Item "None" of "Union[ndarray, Any, None]" has no attribute "dtype"
pandas/core/window/rolling.py:250: error: Incompatible return value type (got "Optional[ndarray]", expected "ndarray")
pandas/core/window/rolling.py:448: error: Argument 3 to "_get_roll_func" of "_Window" has incompatible type "Optional[ndarray]"; expected "ndarray"
pandas/core/window/rolling.py:788: error: Missing return statement
pandas/core/window/rolling.py:788: error: Return type "ndarray" of "_get_window" incompatible with return type "int" in supertype "_Window"
Found 62 errors in 21 files (checked 805 source files)
```
(difference in numbers accounts for things I fixed locally, sorry for that).

As far as I can tell none of these is directly caused by  missing annotations. Some are just incompatible ([created by MonkeyType](https://github.com/pandas-dev/pandas/issues/28843#issuecomment-539545031)?), other are small correctness issues or already mentioned inheritance problems. Not sure about the rest yet, I just started reviewing these.  @c4f3a0ce if you merge master the Linux py36_32bit build should pass @WillAyd

The mypy errors reported above are not a blocker for this PR.

Is this something we want to do at this early stage? see https://github.com/pandas-dev/pandas/issues/28843#issuecomment-539649208 onwards. I could be convinced otherwise but I think this is premature. I'd rather annotate the entire API internally first before release to public i agree we are not ready to release typing even for external dev use
maybe next release will be @c4f3a0ce Thanks for looking into this.

This effort is definitely not wasted as it provides a good basis for further discussion in #28142

but closing for now.