Set JsonFormatter.__init__ return type to None

[nhairs]

Original Issue: madzak/python-json-logger#173

Hello, according to PEP 484, __init__ methods need to be marked as if they return None.

(Note that the return type of init ought to be annotated with -> None. The reason for this is subtle. If init assumed a return annotation of -> None, would that mean that an argument-less, un-annotated init method should still be type-checked? Rather than leaving this ambiguous or introducing an exception to the exception, we simply say that init ought to have a return annotation; the default behavior is thus the same as for other methods.)

In my code I have a custom formatter that derives JsonFormatter and calls super().__init__, but the lack of types triggers mypy. I can "solve" this on my end by adding a # type: ignore, but it's such a low hanging fruit that it's probably worth adding it here.

I'm willing to make a PR for this