pywikibot/__init__.py: Do not use `deprecate_arg`
pywikibot/__init__.py: Do not use `deprecate_arg`

`deprecate_arg` is deprecated itself and redirected to
`pywikibot.tools.deprecate_arg` which is imported as `__deprecate_arg`.

Rename `__deprecate_arg` to `_deprecate_arg`. Python mangles names that start
with two leading underscores inside classes to make them private. This prevented
the usage of `__deprecate_arg` in Coordinate class.

Bug: T162509
Change-Id: Icf557d5f337c302b328ad784317cdb2934bcf458

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