Page.namespace() shall return Namespace() for pages in Main ns
Page.namespace() shall return Namespace() for pages in Main ns

This reverts commit 5a612064e70653d4a6dd043c5b727d54cb5193c8
and restores f6556636fd91a13394f7f6412593ae29c238b2b3.

Bug: T104864
Change-Id: I2d91ca1a9772d909874102e0e9149aa5bfd96b62

h4ck3rm1k3/pywikibot-core,jayvdb/pywikibot-core,h4ck3rm1k3/pywikibot-core,wikimedia/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,Darkdadaah/pywikibot-core,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,hasteur/g13bot_tools_new,PersianWikipedia/pywikibot-core,npdoty/pywikibot,happy5214/pywikibot-core,hasteur/g13bot_tools_new,jayvdb/pywikibot-core,magul/pywikibot-core,wikimedia/pywikibot-core