MAINT: Make pytest-xdist a dependency

[djarecka]

Since we have pytest in REQUIRES I added also pytest-xdist, since this is now required to run nipype.test() (motivated by #2645)

[djarecka]

it's failing because of #2650

[oesteban]

Why not moving the other way around? (pytest to TESTS_REQUIRES)

[djarecka]

@oesteban - sure, can do other way around. However, if I remember correctly some discussion when I was changing tests, we wanted user to be able to run nipype.test even after the most basic installation. am I wrong?

[oesteban]

You are right. I'm happy either way, just to make sure we are doing this.

[effigies]

Either way is fine with me. Strictly speaking, xdist shouldn't be a requirement, but it does seem to be causing people issues. My preferred option would probably be to disable -n auto if xdist isn't present, but that's probably not trivial.

Given how tightly integrated pytest is at this point, with the fixtures and all, having tests as first class components means it is a dependency. So if we can't decouple xdist, we should probably have it as a normal REQUIRES.

[djarecka]

@effigies I think you can't use -n flag at all if you don't have xdist. We can always check if xdist is available and based on this change the pytest command. On the other hand, I don't know if we have a good reason to avoid xdist in the requirements (I mean we're asking for many outside standard library packages anyway)