Update pre-commit GitHub action

[mxr]

This change ensures that we can run pre-commit in CI.

set-env is now discouraged because of:

Error: Unable to process command '::set-env name=PY::1453f75cdbef9e7a53ef361293b38c015cbc84ebad668b2f8bda87df40335b44' successfully.
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

So instead I am updating the config to mirror the change in pre-commit/action#31

Further, mypy fails with a slew of

unkey.py:26: error: syntax error in type comment
unkey.py:33: error: syntax error in type comment
unkey.py:76: error: syntax error in type comment

This issue is tracked in python/mypy#8872 and updating mypy fixes that issue