Controller.start() is missing type annotation

[cuu508]

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The aiosmtpd library has the py.typed marker (awesome!), but the aiosmtpd.controller.Controller.start() method has no type annotation for the return type. This leads to a mypy warning in calling code (when run with --strict):

path/to/file.py:141: error: Call to untyped function "start" in typed context  [no-untyped-call]

Looking at the source, seems it could be annotated to return None.