Logger.getChild bad signature #6606

The signature of Logger.getChild is

def getChild(self, suffix: str) -> Logger: ...

However, this fails if one subclasses Logger, e.g. the following code

logger = MyLogger(...)
child: MyLogger = logger.getChild(...)

Raises type[assignment] error (expression has type "Logger", variable has type "MyLogger"). The signature should be

def getChild(self: T, suffix: str) -> T: ...

where T = TypeVar('T', bound=Logger). Cf. python/mypy#1212 and python/typing#980