upgrade flake8 to 3.8.4 #36879
closes #36879

Here are the logs after the upgrade to 3.8.4:

```console
(base) root@33d4c5d14a5b:/home/pandas-tazminia# pre-commit run flake8 --all
[INFO] Initializing environment for https://github.com/python/black.
[INFO] Initializing environment for https://gitlab.com/pycqa/flake8.
[INFO] Initializing environment for https://gitlab.com/pycqa/flake8:flake8-comprehensions>=3.1.0.
[INFO] Initializing environment for https://github.com/PyCQA/isort.
[INFO] Initializing environment for https://github.com/asottile/pyupgrade.
[INFO] Initializing environment for https://github.com/pre-commit/pygrep-hooks.
[INFO] Initializing environment for https://github.com/asottile/yesqa.
[INFO] Installing environment for https://gitlab.com/pycqa/flake8.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://gitlab.com/pycqa/flake8.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
flake8...................................................................Passed
flake8-pyx...............................................................Passed
flake8-pxd...............................................................Passed
```
This resolves #36879 Thanks @Tazminia ! pre-commit is the only CI check affected by this change, and that passes, so am merging

`isort` can probably also be upgraded if you fancy doing that @MarcoGorelli, yeah I could do so, should I upgrade it to the latest available version ? 
Also, do you have any idea why this PR did not count on my hacktoberfest profile? all other PRs I have done so far counted. we've not yet decided whether to take part in hacktoberfest

Yes - actually, there's a built-in pre-commit command you can run, ` pre-commit autoupdate` re hacktoberfest - in the end we're marking individual PRs as accepted, so have done so here