Added "Site" to the __all__ variable
Added "Site" to the __all__ variable

Site did not appear in the generated documentation when we used help
(pywikibot). Some IDEs that have intellisense feature depend on the function
to be available in the __all__ variable for suggesting code completion.
Using "from pywikibot import *" did not import Site.

Bug: T122563
Change-Id: I4a1c92a380a443d8a3bd99dc65b9e9f192e43d42

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