update 3 SourceForge bug ids to Bugzilla ones

3 other bugs were closed on SourceForge without being imported
on Bugzilla:
- pywikibot/page.py:1840       bug #1795683
- pywikibot/textlib.py:221     bug #1731008
- scripts/interwiki.py:1900    bug #3081100
(maybe they should be removed from comments)

Change-Id: Iecba7695b828837fdd3c3e91beac8c815ef94825