[setup] add pathlib2 to PY2 dependencies
[setup] add pathlib2 to PY2 dependencies

version.package_version() uses pathlib which is a python 3 library.
pathlib2 is required for python 2.7 then.

Change-Id: I0b9f5a94a30942c66679257e5493000f876a1c7c

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core