Support pytest 4.1.0

[madzim]

My first attempt a PR.

I upgraded to pytest 4.1.0 and ran into the deprecation of the node.get_marker function.

https://github.com/pytest-dev/pytest/issues/4546

[theY4Kman]

Hot damn, man, thank you!

I'm gonna close this in favour of #7, submitted in the interest of releasing a new PyPI version quickly, for these few reasons:

• 7 falls back to Node.get_marker if Node.get_closest_marker does not exist (I'm not actually sure if this is a possibility, but since I'm still at work, I figured I'd just keep compat and not worry about it)

• 7 is more lenient w/ pytest requirements (>=3.6 vs >4.10 — you might've wanted >=4.1.0, btw)

• I haven't documented this anywhere (shame on me), but incrementing the version is done outside PRs. This allows the maintainer (moi) to bundle up many PRs/changes into a single release.

Again, though, thank you very much for this. It's nice to know people are getting some value out of the package, and even more heartwarming to see ya care enough to submit a PR <3

[theY4Kman]

Okies, pytest-only 1.2.1 released on PyPI: https://pypi.org/project/pytest-only/

Lemme know if you have any issues!