TYP: add missing `py.typed` marker
I'd like to use `mypy` to check type hint in my project. But it's failed with following error (apache/iceberg#6254)
![image](https://user-images.githubusercontent.com/6848311/203572069-1bd0d889-e081-447a-b43a-a95a494d29e1.png)

`pandas` seem support for type hint already, however, because of missing [`py.typed` marker](https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports), `mypy` can't analyze this package and report the error.
xref https://github.com/pandas-dev/pandas/issues/28142. cc @Dr-Irv on whether we're ready to add this or not yet. We should not do this.  If people want to get type checking with pandas, they should install `pandas-stubs` > We should not do this. If people want to get type checking with pandas, they should install `pandas-stubs`

thanks @Dr-Irv - closing then, but thanks @dungdm93 for your PR Thanks @Dr-Irv, but is their any reason why we should not use builtin type checking? > Thanks @Dr-Irv, but is their any reason why we should not use builtin type checking?

There are a few reasons:
1) The types inside of pandas are not complete.
2) The types inside of pandas are meant more for internal type checking of the code, and less for users of pandas. 
3) Many of the API functions are not typed in the pandas source.

That's why we created pandas-stubs.  It's meant to support type checking of user code, and we can do a LOT more checking there than is possible within the pandas code itself.  We also have a testing mechanism that tests the validity of those type stubs.

Install pandas-stubs and use it.  You'll be glad you did!
