• Add typing information to Plugin.session
• Implement http_session stub file due to the HTTPSession subclass
  of requests.Session which adds additional keywords to the
  request() method, including all other HTTP-verb methods
• Add flake8-pyi and typing_extensions to dev-requirements.txt
  (typing_extensions is not a runtime dependency)

With the removal of the Plugin.bind() classmethod in #4768, now that the Streamlink session instance gets set on each Plugin instance, typing information can finally be added, as the session attribute doesn't have to be defined as None anymore initially.

Adding typing informations finally allows plugin implementors to see all the stuff that's defined on the Session, including the HTTPSession via self.session.http. However, since Streamlink subclasses requests's Session and adds additional keywords to the request() method and thus also to all HTTP-verb methods like get(), post(), etc., this introduces typing issues. The most common problem by far is of course the addition of the schema keyword, which also changes the return type of the various methods from Response to Any.

I originally tried to avoid adding a stub file and directly adding the missing HTTP-verb methods to the HTTPSession subclass and adding typing information there, but that got very messy. The repetitive typing information therefore needs to be defined in a stub file. See PEP561.

Unfortunately though, it doesn't seem to be possible to import already defined type aliases from the types-requests package, so I had to copy all required definitions for the other args+keywords provided by requests.

The typing_extensions dependency had to be added because mypy is configured for python 3.7, and TypeAlias wasn't part of the typing module yet in 3.7. Otherwise a error: Module "typing" has no attribute "TypeAlias" would be raised.

The new Y037 linting error also has to be ignored because of an apparent mypy bug (error: Type application has too many types (1 expected)) when union types like a | b | c are defined instead of Union[a,b,c]. This is all a bit weird, because the copied type alias defintions are using both styles. Maybe it's because of the re-defintions... The union types syntax in stub files is valid on Python 3.7, btw:
https://typing.readthedocs.io/en/latest/source/stubs.html#syntax

Opening this as a draft for now.