Disable F821 and F822 by default

[AlexWaygood]

This PR proposes disabling the troublesome F821 error code ("undefined name name") by default. It also fixes F822 ("undefined name name in __all__") so that variables that are annotated, but not assigned to, are recognised by flake8-pyi as being defined in the file. Implementing these changes will allow us to remove the remaining = ...s at the module level in typeshed.

I initially tried to implement this using the extend_default_ignore option that flake8 provides. However, this was not a good solution. If a user specifies --per-file-ignore options in their .flake8 config file, they will need to manually add F821 or F822 to each subdirectory, as specifying these options in a config file overrides the list of ignored-by-default error codes. Moreover... we're already monkeypatching pyflakes in this plugin, so a little more monkeypatching probably won't hurt too much. Lastly, this solution has the advantage that it only affects how .pyi files are parsed by flake8.

[JelleZijlstra]

This makes me worried that we'll suppress these errors in .py files too. I think that won't happen because we monkeypatch only for pyi files, but would it be possible to add a .py test case to make sure we don't break linting on .py files?

[AlexWaygood]

Okay, anybody know how to get the CI flake8 check to exclude the new .py file that deliberately has flake8 errors? 🙃

[AlexWaygood]

I'm closing this since, as discussed in #183, we should be reducing the amount of monkeypatching we do, not adding to it.

This PR doesn't really add to the problem massively, but it's also not a long-term solution, so I don't think it's worth it for now.