Allow covariants of __enter__, __aenter__, and @classmethod

[Congee]

The problem we currently have is the return type of classes such as
Client does not allow covariants when subclassing or context manager.
In other words:

classBase:
def__enter__(self) ->Base: # XXXreturnselfclassDerived(Base):
...
withDerived() asderived:
# The type of derived is Base but not Derived. It is WRONG
...

There are three approaches to improve type annotations.

1. Just do not type-annotate and let the type checker infer
   return self.
2. Use a generic type with a covariant bound
   _AsyncClient = TypeVar('_AsyncClient', bound=AsyncClient)
3. Use a generic type T = TypeVar('T') or Self = TypeVar('Self')

They have pros and cons.

1. It just works and is not friendly to developers as there is no type
   annotation at the first sight. A developer has to reveal its type via
   a type checker. Aslo, documentation tools that rely on type
   annotations lack the type. I haven't found any python docuementation
   tools that rely on type inference to infer return self. There are
   some tools simply check annotations.

2. This approach is correct and has a nice covariant bound that adds
   type safety. It is also nice to documentation tools and somewhat
   friendly to developers. Type checkers, pyright that I use, always
   shows the the bounded type '_AsyncClient' rather than the subtype.
   Aslo, it requires more key strokes. Not good, not good.

   It is used by BaseException.with_traceback
   See https://github.com/python/typeshed/pull/4298/files

3. This approach always type checks, and I believe it will be the
   official solution in the future. Fun fact, Rust has a Self type
   keyword. It is slightly unfriendly to documentation, but is simple to
   implement and easy to understand for developers. Most importantly,
   type checkers love it.

   See SelfType or another way to spell "type of self" (or, How to define a copy() function) python/mypy#1212

But, we can have 2 and 3 combined:

_Base=typing.TypeVar('_Base', bound=Base)
classBase:
def__enter__(self: _Base) ->_Base:
returnselfclassDerive(Base): ...
withDerived() asderived:
... # type of derived is Derived and it's a subtype of Base

[Congee]

Hi, this PR is a follow up to #1333 . This PR introduces a typing problem for StreamContextManager, it returns a httpx.Response. I am not sure how to type it. How would you like it to be typed?