[WIP] Fix type annotations for Pandas.core.indexes.datetimelike
- [x] closes #26364 
- [ ] tests passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Errors that still remain are:

    pandas\core\indexes\datetimelike.py:66: error: "Callable[[Any], Any]" has no attribute "fget"
    pandas\core\indexes\datetimelike.py:67: error: "Callable[[Any], Any]" has no attribute "fget"
    pandas\core\indexes\datetimelike.py:68: error: "Callable[[Any], Any]" has no attribute "fget"
    pandas\core\indexes\datetimelike.py:70: error: "Callable[[Any], Any]" has no attribute "fget"
    pandas\core\indexes\datetimelike.py:71: error: "Callable[[Any], Any]" has no attribute "fget"
    pandas\core\indexes\datetimelike.py:208: error: Name '_box_values' already defined on line 73

Errors related to `fget` doesn't seem to have any solution yet(see https://github.com/python/mypy/issues/6185 and linked mypy issues in its comments). Maybe we should ignore it for now.

@WillAyd need some insight on why `_box_values` is declared twice and how to annotate it, or maybe ignore it too.

btw mypy tests will fail, so maybe someone with access to azure pipelines should cancel the tests to allow tests from PR(if any) to run. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26365?src=pr&el=h1) Report
> Merging [#26365](https://codecov.io/gh/pandas-dev/pandas/pull/26365?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b48d1ff87237cb68792590f4377e3be5c73d5ea7?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26365/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26365?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26365      +/-   ##
==========================================
- Coverage   91.68%   91.68%   -0.01%     
==========================================
  Files         174      174              
  Lines       50700    50701       +1     
==========================================
- Hits        46486    46483       -3     
- Misses       4214     4218       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.18% <100%> (ø)` | :arrow_up: |
| #single | `41.17% <100%> (-0.18%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26365?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/26365/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.14% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26365/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26365/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.01% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26365?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26365?src=pr&el=footer). Last update [b48d1ff...a817365](https://codecov.io/gh/pandas-dev/pandas/pull/26365?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 There is already an open PR for this one so closing this as a duplicate my bad, didn't saw that