Missing dependency typing_extensions

When I run black against python 3.9.1



I get the following



```

Run pipenv run black --check .

  pipenv run black --check .

  shell: /bin/bash -e {0}

  env:

    pythonLocation: /opt/hostedtoolcache/Python/3.9.1/x64

Traceback (most recent call last):

  File "/home/runner/.local/share/virtualenvs/meerkat-pop4KNWc/bin/black", line 5, in <module>

    from black import patched_main

  File "/home/runner/.local/share/virtualenvs/meerkat-pop4KNWc/lib/python3.9/site-packages/black/__init__.py", line 45, in <module>

    from typing_extensions import Final

ModuleNotFoundError: No module named 'typing_extensions'

```



**To Reproduce** Steps to reproduce the behavior:



1. Take this file '...'

2. Run _Black_ on it with these arguments '....'

3. See error



**Expected behavior** A clear and concise description of what you expected to happen.



**Environment (please complete the following information):**



- Version: [e.g. master]

- OS and Python version: [e.g. Linux/Python 3.7.4rc1]



**Does this bug also happen on master?** To answer this, you have two options:



1. Use the online formatter at https://black.now.sh/?version=master, which will use the

   latest master branch.

2. Or run _Black_ on your machine:

   - create a new virtualenv (make sure it's the same Python version);

   - clone this repository;

   - run `pip install -e .`;

   - make sure it's sane by running `python -m unittest`; and

   - run `black` like you did last time.



**Additional context** Add any other context about the problem here.

[]