…possible.

Currently, `inspect.getdoc()` fails to inherit docstrings in dynamically
generated subclasses, such as
```
class Base:
    def method(self): "some docstring"

def make_subclass():
    class subclass(Base):
        def method(self): return super().method()
    return subclass

subclass = make_subclass()
inspect.getdoc(subclass.method)  # => returns None
```
because `inspect._findclass()` tries to find the base
class by parsing `subclass.method.__qualname__` which is
`"make_subclass.<locals>.subclass.method"` and chokes over
`.<locals>.`.

In the case where the method does rely on `super()`, there is another
way we can go back to the "owning" class of the method: by looking up
the contents of the `__class__` cell (which is set up to make 0-arg
super()).  This approach is implemented by this PR.

Perhaps a `__class__` cell could even be set up (in a separate patch)
for *all* methods defined in dynamically created subclasses (i.e. whose
`__qualname__` includes `.<locals>.`), to help with introspection?