pwb.py: mock pywikibot if it is not available

run_python_file() can only be called after pywikibot has been loaded.
However, it is impossible to load pywikibot if a user-config.py is
not yet available. We therefore now mock pywikibot, by creating
and object (lambda function) with argvu attribute.

Bug: 56324
Change-Id: If0a1e08e29da3a3780b0b9300fcad140ae7087c5