Show generic arguments

[sobolevn]

Hi again! 🙂

In my use-case, it is very important to show what generic arguments types have.
Let's see it with real-life example.

This is how my Maybe is show in the new version of this plugin:

But, that's how it is defined:

class Maybe(
    BaseContainer,
    SupportsKind1['Maybe', _ValueType],
    MaybeBased2[_ValueType, None],
    metaclass=ABCMeta,
):

• https://github.com/dry-python/returns/blob/master/returns/maybe.py#L34-L39
• https://github.com/dry-python/returns/blob/master/returns/interfaces/specific/maybe.py#L115-L119
• https://github.com/dry-python/returns/blob/master/returns/interfaces/container.py#L100-L105

See, how many information about typing is lost right now! 😞

I propose to show all available typing information if :generics: key is given.

Here's a quick demo to show that it is possible.

>>> from typing import Generic
>>> from typing import TypeVar
>>> T = TypeVar('T')
>>> class Example(Generic[T]): ...
... 
>>> Example.__mro__
(<class '__main__.Example'>, <class 'typing.Generic'>, <class 'object'>)
>>> Example.__orig_bases__
(typing.Generic[~T],)

Related to #48