Add N808 for type variable name convention check

Also upgrade some GitHub actions to their latest versions.

Python 3.8 reached the end of its supported life on 2024-10-07.

https://devguide.python.org/versions/

Python 3.6 introduced __init_subclass__(cls), which provides a nicer
subclass registration pattern than the previous metaclass-based
approach.

We previously constructed a frozen set of names that should be ignored
at setup-time and then passed that set as an argument to each visitor
function. A helper function -- _ignored(name, set) -- was used to test
whether a given name should be ignored, matched using fnmatchcase().

This change introduces a `NameSet` type, which inherits from frozenset
and implements `__contains__` using fnmatchcase(). This allows us to
simply use the `in` operator in visitor functions.

The previous code also (sometimes) treated the `ignores` argument as
optional (defaulting to None). This value always exists so remove the
optional notation.

When the set of ignored names doesn't use shell-style wildcards, we can
use the faster `frozenset.__contains__` base implementation rather than
the more expensive iteration-based fnmatch'ing approach. This is true
for the default ignore list, and I expect its the more common case by
far even for those who add their own ignored names.

In a simple local benchmark, this results in a 2x speed improvement for
that common (default) path, which I think justifies the small additional
complexity.

This allows it to be used with reversed(). We could also have used
Reversible, but Sequence is the more natural base of deque.

First, represent FunctionType as an enum.Enum. This is more about the
data type's concept than any higher-level Enum features.

Then use FunctionType so improve the typing of various dictionaries and
function signatures.

Lastly, use setattr() for our AST monkey patching. I tried a few other
approaches here (using ast.(Async)FunctionDef subclasses, introducing a
new protocol, etc.), but the monkey patching approach always came out
the simplest and most obvious. Using setattr() avoids typing errors
related to unknown direct attribute assignments.