Add final declarations to the library constants.

[asvetlov]

The library has many constant definitions that are declared as NAME = 'val', e.g. https://github.com/aio-libs/aiohttp/blob/master/aiohttp/client.py#L161

We need to add Final type hint to let users know that these constants should be considered as immutable (this hint doesn't prevent the actual modification and fully backward compatible).

from typing_extensions import Final  # support Python < 3.8

NAME: Final = 'val'

[WisdomPill]

Hello @asvetlov !

I would be glad to step in and make a PR, are there other places that come to your mind that have these kind of constants?

[asvetlov]

Basically, any module-level assignment falls into one of three categories:

1. __all__ = ... Not sure what to do. Maybe nothing.
2. Type alias, e.g. _BytesIterator = AsyncIterator[bytes]. Nothing to do here.
3. Constant assignment. aiohttp has no global variables at all IIRC but has very many global constants. aiohttp/hdrs.py is a good example.

[WisdomPill]

Closing, it was merged almost a month ago