Allow failures on Pypy3

[brandur]

Pypy3's been breaking pretty regularly due to an import error:

ImportError: cannot import name 'pythonapi'
make: *** [init] Error 1

Here we (temporarily) allow it to fail in the build matrix so that we
can get the general build back to green.

r? @ob-stripe This is obviously not great, but a failing master build
isn't either :) What do you think about bringing this in temporarily and
trying to get the Pypy3 build fixed later? It's not too obvious to me
why it's failing -- I'd have to spend some time digging in.

[ob-stripe]

Yeah, it's annoying. I fixed the pipenv issue with PyPy 3 a while back (pypa/pipenv#3322), but they haven't released any new version since then.

Mind adding a comment to explain this is just a pipenv/PyPy3 compatibility issue? Feel free to merge afterwards.

[brandur-stripe]

Oh, right! Now that you mention it, I remember you talking about that. That is annoying :)

I added a comment explaining that it should be possible to remove Pypy3 from allow_failures soon with a link to your PR. Thanks!

[jleclanche]

@brandur-stripe @ob-stripe A bit crass but you could apply the patch after pipenv is installed but before it's run on Travis. This would allow the build to run at least (and who knows if there's other issues in pipenv with pypy)

[brandur-stripe]

@jleclanche Great point! My general feeling is that it's probably not worth it right now in that if we just wait a bit longer for a release we can save ~an hour's worth of work and spend it elsewhere. Let's revisit that if the release turns out to be a really long time coming though — I'm not opposed to giving the idea a shot at that point.

[ob-stripe]

Alternatively, we could pin pipenv to a previous version since the PyPy3 incompatibility was introduced in a recent release.

[brandur-stripe]

@ob-stripe Yep. Up to you on that one! Might be worth it if you know you can do it quickly.