Introduce InterwikiRedirectPage

Interwiki links in redirects will raise InterwikiRedirectPage instead
of CircularRedirect exception.

Interwiki redirects are considered an invalid page by Pywikibot due
to limited support for this kind of redirects on Mediawiki.
The API can't follow redirects if the redirect is an interwiki link.

Also fixed:
- uniform importing of exceptions in site.py
- pep257 in pywikibot.exceptions

Bug: 73184
Change-Id: Ia0d4dadf713fb97572c5d482485858331bda5ea8
Merge "Introduce InterwikiRedirectPage"