Allow callables in _SourceObjectType (#3295)

This is important because mypy doesn't generally think functions are compatible with `FunctionType`, so `inspect.getsource` on arbitrary functions is rejected by the current annotations.

stdlib/3/inspect.pyi

@@ -77,7 +77,7 @@ def ismemberdescriptor(object: object) -> bool: ...
 #
 # Retrieving source code
 #
-_SourceObjectType = Union[ModuleType, Type[Any], MethodType, FunctionType, TracebackType, FrameType, CodeType]
+_SourceObjectType = Union[ModuleType, Type[Any], MethodType, FunctionType, TracebackType, FrameType, CodeType, Callable[..., Any]]
 
 def findsource(object: _SourceObjectType) -> Tuple[List[str], int]: ...
 def getabsfile(object: _SourceObjectType) -> str: ...