PEP 561 compliance

[tomwatson1024]

Add type hints and a 'py.typed' marker file (see PEP 561) to allow type checking in dependent code.

I've not added any automatic type checking or anything - I have run mypy over the code to confirm that my type hints are correct, though.

Not sure if this qualifies for a news fragment - happy to write one if necessary.

[codecov]

Codecov Report

Merging #17 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           72        73    +1     
  Branches         6         6           
=========================================
+ Hits            72        73    +1     

Impacted Files	Coverage Δ
sniffio/_impl.py	100.00% <100.00%> (ø)

[sethmlarson]

Thanks for this! Two general comments:

• I believe there's an extra step you need to do to make sure py.typed shows up in our dists, maybe add to MANIFEST.in?
• Within ci/travis.sh can you add a step in CHECK_FORMATTING that runs mypy on the file?

And I don't think a newsfragment is necessary, this project doesn't require a ton of updates to be coordinated :)

[tomwatson1024]

* I believe there's an extra step you need to do to make sure `py.typed` shows up in our dists, maybe add to `MANIFEST.in`?

Ah yes, of course - I've gotten used to Poetry, which just automagically makes this kind of thing work!

* Within `ci/travis.sh` can you add a step in `CHECK_FORMATTING` that runs mypy on the file?

Can do.

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[sethmlarson]

Thanks for these changes! Will prepare a 1.2.0 release :)