Add a __class_getitem__ to Formatter

[Akuli]

This is a simpler and less disruptive alternative to #2081.

The type stubs in typeshed distinguish between formatters that output strings and bytes. In type annotations, these are distinguished with Formatter[str] and Formatter[bytes]. For the most part, this works fine without any changes in pygments: users just tell Python to not evaluate type annotations at runtime (with from __future__ import annotations), and then a Formatter[str] type annotation won't cause an error even though subscripting the Formatter class wouldn't work.

But one special case happens when you try to subclass a Formatter. For that, you actually need to specify a class, not a type annotation. So users end up with:

from typing import TYPE_CHECKING
from pygments.formatter import Formatter

if TYPE_CHECKING:
    StringFormatter = Formatter[str]
else:
    StringFormatter = Formatter

class MyFormatter(StringFormatter):
   ...

With this PR, this simplifies to:

from pygments.formatter import Formatter

class MyFormatter(Formatter[str]):
   ...

[Anteru]

Merged, thanks!