Allow pywikibot to load without a user-config.py

Set environment variable PYWIKIBOT2_NO_USER_CONFIG=1
to allow the library to load without a user-config.py

Emits a warning to help triage problems with this
functionality.

Change-Id: I8f77f7634989bef75701ed5cf952844e339c1243
Merge "Allow pywikibot to load without a user-config.py"