Introduction
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rename `pagegenerators.GeneratorFactory.handleArg()` method to `handle_arg()` due to PEP8 naming convention
What to do:
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[x] rename `handleArg()` method of `pagegenerators.GeneratorFactory` to `handle_arg()`
[x] deprecate the old method with `deprecated` Decorator (from `pywikibot.tools`)
[x] replace all occurences of this GeneratorFactory.handleArg() with the new identifier. They can be found [[https://codesearch.wmcloud.org/pywikibot/?q=handleArg%5C(&i=nope&files=&repos= | here]]
Important hints
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- be aware: `GeneratorFactory` method is `handleArg()` not `handleArgs()` because it processes one option only
- a similar deprecating solution can be found for `handle_args()`/`handleArgs()` function in `pywikibot.bot` module
- refer [[https://www.mediawiki.org/wiki/Manual:Pywikibot/Development#Guidelines | our Coding Convention Guidlines]]