Add support for positional only arguments

[pganssle]

PEP 570 adds "positional only" arguments to Python, which changes the
code object constructor. This adds support for Python 3.8.

I assume this is covered by existing tests, but it would be good to add a "nightly" build to the CI matrix.

Fixes #1943

[Zac-HD]

Thanks @pganssle - this looks great to me! Three things before I can merge this:

1. black would like "-quoted strings here.

2. For our automatic deployment to work, we'll need a changelog file (hypothesis-python/RELEASE.rst). I suggest the following content:

   RELEASE_TYPE: patch
   
   This patch makes Hypothesis compatible with the Python 3.8 alpha, which
   changed the representation of code objects to support positional-only 
   arguments.  Note however that Hypothesis does not (yet) support such 
   functions as e.g. arguments to :func:`~hypothesis.strategies.builds`
   or inputs to :func:`@given <hypothesis.given>`.
   
   Thanks to Paul Ganssle for identifying and fixing this bug.
   

3. You're also invited to add your name to the alphabetised list of authors in CONTRIBUTING.rst.

Then I'll merge ASAP - and hope to see you at PyCon!

[pganssle]

Sure thing. I can do that in an hour or so.

This is also slightly tricky because it will break anyone testing with earlier alpha releases that don't include PEP 570. If that's not a concern we can leave as is, but if it is a concern, I can try and come up with a way to detec PEP 570 support and guard on that.

[Zac-HD]

I'd prefer to keep it as simple as possible, so this form looks great 😄

(plus the string formatting thing noted above)

[pganssle]

Hm.. I'll rebase this once the previous release is done. I guess that's a problem with this system of "every PR triggers a release" (though presumably just a problem until the release bot or whatever removes RELEASE.rst from master.

[Zac-HD]

Yep, sorry about that! Just a ~15 minute window where pushes get merge conflicts.

We've looked at a bors-like system but haven't quite needed it yet... though it would help at sprints.

[pganssle]

Thanks @Zac-HD, see you at PyCon!