Convert function decorators to class decorators

[Peilonrayz]

Hey, sorry for the delay.
I have improved typing as discussed in #287. I have:

• Updated the noxfile.py to include the --warn-unused-ignores flag.
• Made a _decorators file, as the code started to call and inherit from multiple different files. This was causing some cyclical dependencies.
• Changed _parametrize.generate_calls to a class decorator, Call.
• Changed registry.session_decorator to delegate to a class decorator, Func.
• Moved manifest._copy_func into _decorators and call it in Func.
• Changed manifest.Manifest.make_session to use Func.copy rather than manifest._copy_func.
• Made Call subclass Func.
• Removed a large amount of # type: ignore.
• Updated a few Callable to Func.
• Updated tests.

Given the size of the existing PR I have decided to leave the typing of the attributes as is.

[theacodes]

Great work! Thank you so much!