add type annotations and rework some None/unset/default cases

[pajod]

Apparently we can ship type annotations in separate *.pyi files right now, with no need to await dropping 3.7 compat and no obvious harm to people that have no use for them. Reworking #2377 I ended up with a certainly still imperfect, but already somewhat useful set of annotations. A few default settings in gunicorn popped out as odd:

• setting --config="" does not disable reading configuration. the next best thing, setting --config=/dev/null loudly complains
• enabling --reload has non-obvious side-effects in error handling, and setting --reload-extra= is silently ignored when set in isolation
• settings that take a callable put the staticmethod in the next line instead of as a decorator. this way static checkers cannot tell the default callables are never meant to receive the usual self method arg.
  • refactoring this requires a minor fix in docs/gunicorn_ext.py to keep documentation unaffected
• uid/gid settings default to trying even when not expecting any changes.. is this needed? the only effect appears to be breaking some WSL corner cases (where thats not a no-op but a hard error)
• gid = abs(gid) & 0x7FFFFFFF <= now unnecessary and broken for systems that actually use group id > 2**32 (see gunicorn/util.py set_owner_process Mangling gid on RHEL8 + Google Cloud Platform (very large int group id) #3212)
• setting --gid=number without setting --uid=number is.. surprising. There is still that root group in the supplementary group list. Looks like Debian did not like this and patched it - though I would probably branch the initgroups case.
• pytest-cov does not mix well with running just coverage run -m pytest (telling me no coverage was collected..) - is it even needed for anything other than invocation brevity, which is already dealt with by inclusion in tox & Makefile?
• There is a str/bytes type confusion in the gevent_pywsgi worker class (but this was already noticed independently, see Access log formatting is wrong when using gevent_pywsgi worker class #2940)

This is primarily an invitation to discuss potential default settings changes, the attached patches serve as clarification.

Advantages of shipping type annotations:

• highlight oddities in current behaviour stemming from ambiguous/overloaded methods
• can be checked in CI: automatically receive notification before introducing potential type confusion bugs
• assists in understanding existing code, even where currently little to no documentation exists
• assists in reviewing changes which incorporate type changes, easing complex rebase work of outstanding PRs
• helpful in extracting a list of items to be touch when planning to reclassify previously public/unspecified API into private
• enhances IDE features such as quickly navigating to definitions of types or method arguments

Disadvantages of shipping type annotations:

• people submitting new PRs without updating annotations may see less self-explanatory feedback from CI runners
• as long as any annotation is incorrect or incompatible with a popular tool such as mypy, expect some people to show up and report those bugs in annotations at a time where there already are quite a lot open bugs/PRs

Related

Fixes: #2833 #2374 #1393

[pajod]

Naming pattern clash. Sounds like Server Hook but is not implemented as such.