ENH: vendor typing_extensions
- [x] closes #34869
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

haven't figured out how to make isort and black ignore this file
Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/_vendored/typing_extensions.py`](https://github.com/pandas-dev/pandas/blob/1b29450597510adb80c8b29cfadecdfc1c86dce2/pandas/_vendored/typing_extensions.py):

> [Line 1985:89](https://github.com/pandas-dev/pandas/blob/1b29450597510adb80c8b29cfadecdfc1c86dce2/pandas/_vendored/typing_extensions.py#L1985): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (98 > 88 characters)



##### Comment last updated at 2020-09-01 16:25:26 UTC cc @WillAyd @simonjayhawkins thoughts on getting this to pass checks? FWIW I think OK to run the new module through isort / black; doesn't need to be a perfect reproduction

AFAICT code check failures are mostly around internal style rules we place; OK to update to those as well as they are mostly cosmetic > do we need any sort of testing for this?

I wouldn't bother.  FWIW not needing to worry about that is part of why i suggested making this a dependency in #34869 thanks @jbrockmendel 