Add @final to many unsubclassable stdlib classes

[AlexWaygood]

These were all discovered using the following extremely crude script, the output of which I then manually checked:

import sys
from inspect import isclass
from enum import EnumMeta

unsubclassables = set()
for modname in sys.modules:
    mod = __import__(modname)
    for thingname in dir(mod):
        thing = getattr(mod, thingname)
        if isclass(thing) and not isinstance(thing, EnumMeta):
            try:
                class Foo(thing): pass
            except TypeError:
                if thing not in unsubclassables:
                    print(modname, thingname)
                    unsubclassables.add(thing)

The only results that this script produced that I have not added @final to are dict_keys, dict_values and dict_items (exposed in collections.abc at runtime, and in builtins in typeshed). I couldn't add @final to these classes because of the following weirdness:

>>> from collections import OrderedDict
>>> dict_keys = type({}.keys())
>>> ordered_dict_keys = type(OrderedDict().keys())
>>> class Foo(dict_keys): pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type 'dict_keys' is not an acceptable base type
>>> issubclass(ordered_dict_keys, dict_keys)
True

So, while these classes are not subclassable at runtime, they nonetheless have subclasses in the stdlib, meaning mypy understandably complains if I try to add @final to them.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉