tox: invoke diff_checker directly
tox: invoke diff_checker directly

The diff checker maintenance is invoked using pwb.py however that
requires requests to be installed for pwb. Just invoke the script
directly.

Change-Id: If42437452fd7214612153b7fe481689b8ea8aef9

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