Add typing stubs

[bluetech]

There a couple preparatory commits:

• Mark the wheel universal -- The code works on both Python 2 and 3 so the same wheel can be used for both.

• Use a package layout, src/ and testing/ directories - needed for the py.typed file

The last commit adds the typing stubs. This is copy/pasted from the stubs of the py.iniconfig module in the py library, however I removed the _ prefix from SectionWrapper type since it's good to be able to refer to it. I marked its __init__ as private (there is unfortunately no programmatic way to mark this).

Tested working against pytest's code base (there are a couple legitimate errors I would have to fix there).

[bluetech]

@RonnyPfannschmidt updated to use testpaths.

Also added a commit "tox: remove junit generation" which changes the command just to pytest, I don't think anything is using the junitxml stuff here. Can remove it though if it is used by something.

[RonnyPfannschmidt]

Thanks!

[bluetech]

Thanks for reviewing and merging @RonnyPfannschmidt.

A release would be great once you have time. It would break pytest linting, I will send this fix then.