Add typing to ParserMethod

[ribetm]

Assumes eager=False: parsers are shown to return their base type and neither Any nor None.

I think this should be fine as users should call env.seal() and either terminate or at least not use invalid values in case of errors.

[ribetm]

I tried to fix #120 while keeping maintenance cost as close to zero as possible and compatibility with Python 3.8.

Any upgrade of the targeted version would help simplifying the type hints greatly, but it is already functional as is.

[sloria]

Thanks for the PR!

Not sure it's a good idea to assume that the return type for the methods will be non-None, esp. when allow_none can be set to True. is there a good way to handle that?

[sloria]

non-blocking nit: maybe should make this 'private' if it's not meant to be used

Suggested change

	class ParserMethod(typing.Generic[_T]):
	class _ParserMethod(typing.Generic[_T]):

[ribetm]

I don't think it's possible without @overload which has already been suggested before.

I guess the best we can do is combining the overloads from #120 with the shorter Generic syntax. I added a commit to do that.

[sloria]

looks like there's still some mypy errors in CI--can you take a look at those?

[nfantone]

Found this while looking to add type hints to environs parsed variables. Having this would be great!

@sloria @ribetm Can I help here?