Add typehinting via py.typed

[piercefreeman]

I'm using pglast in some downstream projects and native typehinting support in my IDE would be pretty convenient. Happy to put up a PR for this functionality but wanted to check in on appetite before I do @lelit.

[lelit]

Hi, I"m taking a short break... Yes, I will appily merge your PR, especially if it comes with basic tests 😉 Bye, lele.

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[DeoLeung]

+1, type hint will be very helpful

[lelit]

I'm sorry, I will try to revive the work done by Pierce.

[lelit]

Hi, working on the merge, I see the Union[str, slice] syntax: what does that mean?

My Emacs setup signals as wrong the stmts: Tuple[str, ...] = split("foo;bar") in the test...

[lelit]

Sorry, ignore my last message above, not enough caffeine I guess ;-)

I merged @piercefreeman 's PR #178 and added some more refining commits on top.

I know it's tempting to spend inconsiderable time improving type hints, but I will consider improving them on the forthcoming v8 branch, where I will drop support for Python 3.9.

[lelit]

Sigh, other changes that I committed on top of that caused a failure on macOS that I cannot investigate, so won't be able to do a proper release for a while.