use setuptools.parse_version in _version_check

[craigahobbs]

This eliminates the dependency on packaging.

[alex]

Jenkins, ok to test.

[alex]

@dstufft can you please tell us what to do here :-)

[alex]

@craigahobbs what's the motivation here? Modern setuptools depends on packaging so it's not like this adds a new dependency.

[dstufft]

I think that the only benefit to this is old setuptools and possibly future setuptools if we convince jaraco to re-bundle his deps. Whether that's worth it or not I dunno, but besides that the change looks fine on the surface.

[alex]

Oh. I just realized, a downside of this is pkg_resources is super slow to import.

[dstufft]

We already use pkg_resources to iterate the list of installed backends though right?

[alex]

We moved that import into the function, so it's only imported when you call default_backends.

[alex]

_version_check is called at module scope, so there's no equivilant solution.

[dstufft]

Ah right.

[craigahobbs]

Regarding motivation, it is solely to eliminate the dependency on packaging. Not a huge deal. We're using Python 3.6 and use the setuptools included with Python. Interestingly, it seems to include packaging - it imports it like this "from pkg_resources.extern import packaging" - not that I'd recommend that.

[reaperhulk]

Yeah we moved to depend directly on packaging to avoid the import time side effects of pkg_resources. So we can't switch back without causing a regression. Sorry, but thanks for the contribution anyway! It's definitely non-obvious why we're doing it this way.

refs #3347 and #3190