Type the Loop base class as generic

[carmocca]

What does this PR do?

Signal that the return type of run is the same as the return type of on_run_end.

Minimal example:

from dataclasses import dataclass
from typing import TypeVar, Generic

T = TypeVar("T")


class Base(Generic[T]):
    def run(self) -> T:
        return self.end()

    def end(self) -> T:
        ...


@dataclass
class Foo:
    ...


class Child(Base[Foo]):
    def end(self) -> Foo:
        ...

without this, when result = self.a_loop.run(...), mypy doesn't know the type of result even if its annotated in the on_run_end signature.

Does your PR introduce any breaking changes? If yes, please list them.

None

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[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9418 (7b8cb6e) into master (e0f2e04) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9418   +/-   ##
======================================
  Coverage      93%     93%           
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  Files         179     179           
  Lines       14922   14923    +1     
======================================
+ Hits        13861   13862    +1     
  Misses       1061    1061