[bugfix] Fix deprecation warning for namespace shortcuts
[bugfix] Fix deprecation warning for namespace shortcuts

namespaces.CONSTANT gives the Namespace object not the string.
Assigning Namespace to a string adds a colon and prepends one for
FILE and CATEGORY Namespace because Namespace.__str__ uses the
canonical_prefix method whereas the old shortcuts gives the default
localized name which is custom_name property. The deprecation warning
should follow this issue.

Change-Id: Idaa5e28dccbdd0311b617ec8607c4bbc96d88fa7

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