Add type annotations to `fontshell`.

[knutnergaard]

Includes:

• annotation of all fontshell modules
• related fixes in base classes

[knutnergaard]

@benkiel See issues #824 and #825.

[knutnergaard]

@benkiel All modules in fontshell (except for the test module) are now annotated.

I've added a method to many of the modules to handle a potential non-value from the naked object. It will look something like:

def _getNaked(self) -> defcon.Info:
        info = self.naked()
        if info is None:
            raise ValueError("Info cannot be None.")
        return info

I'm not sure if the error message is sufficiently clear, so any for changes here are welcome.