Put Flake8 requirements into their own file (#49032)
Summary:
This PR moves the list of Flake8 requirements/versions out of `.github/workflows/lint.yml` and into its own file `requirements-flake8.txt`. After (if) this PR is merged, I'll modify the Flake8 installation instructions on [the "Lint as you type" wiki page](https://github.com/pytorch/pytorch/wiki/Lint-as-you-type) (and its internal counterpart) to just say to install from that new file, rather than linking to the GitHub Actions YAML file and/or giving a command with a set of packages to install that keeps becoming out-of-date.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49032
Test Plan:
Either look at CI, or run locally using [act](https://github.com/nektos/act):
```sh
act -P ubuntu-latest=nektos/act-environments-ubuntu:18.04 -j flake8-py3
```
Reviewed By: janeyx99
Differential Revision: D25404037
Pulled By: samestep
fbshipit-source-id: ba4d1e17172a7808435df06cba8298b2b91bb27c
