Support ParamSpec

[sobolevn]

Currently we use a custom mypy plugin to fix typings of some decorators.
Plugin: https://github.com/dry-python/returns/blob/master/returns/contrib/mypy/_features/decorators.py

Progress: python/mypy#8645
PEP: https://www.python.org/dev/peps/pep-0612/
Source: https://github.com/python/cpython/blob/366dc3a1354078e38808b9c16276e97cca5b8aaf/Lib/typing.py#L730

Example:

from typing_extensions import ParamSpec
from typing import TypeVar, Callable

T = TypeVar('T')
P = ParamSpec('P')

def add_logging(f: Callable[P, T]) -> Callable[P, T]:
    '''A type-safe decorator to add logging to a function.'''
    def inner(*args: P.args, **kwargs: P.kwargs) -> T:
        logging.info(f'{f.__name__} was called')
        return f(*args, **kwargs)
    return inner

@add_logging
def add_two(x: float, y: float) -> float:
    '''Add two numbers together.'''
    return x + y

It is not working right now, we need to wait until proper support is merged into mypy.

[thepabloaguilar]

That's beautiful!!

[thepabloaguilar]

Refs #264