Use pathlib2

The dependency on `pathlib` was added in pytest-dev#631. A backport is needed for Python <3.4.

The `pathlib` package [on PyPI](https://pypi.org/project/pathlib/) is maintenance-only and comes with a message about using `pathlib2` for an up-to-date version, still importable as `pathlib`. Also `pytest` itself uses `pathlib2` so currently I'm getting both packages installed, one on top of the other, woops!

This change in `setup.py` seems to be all that's needed to switch.

setup.py

@@ -32,7 +32,7 @@ def read(fname):
     setup_requires=['setuptools_scm>=1.11.1'],
     install_requires=[
         'pytest>=3.6',
-        'pathlib;python_version<"3.4"',
+        'pathlib2;python_version<"3.4"',
     ],
     extras_require={
         'docs': [