Cleanups for when we drop Python 2 compatibility

[Tim Graham]

This is a tracking ticket of things that we can remove when we drop Python 2 compatibility (in a few years or so). Please edit the description of the ticket as you come across new items.

• django.dispatch.weakref_backports
• django.utils.2to3_fixes
• django.utils.lru_cache
• django.utils.html_parser.use_workaround
• django.utils.decorators.ContextDecorator
• django.utils.six
• Change inspect.getargspec() (deprecated since 3.0) to inspect.getfullargspec()
• @python_2_unicode_compatible
• from __future__ import unicode_literals
• Lots of workarounds in django.http.cookie
• str() stuff for environment variables, e.g. 0bfb53866199f366ed140d49938fd185e5898156
• django.core.mail.message.make_msgid() #23905
• Functions in django.utils.http like urlquote_plus -- I think the versions of these functions on Python 3 don't have the unicode characters bug we are working around.

[Wojtek Ruszczewski]

Various nonlocal workarounds (​example).

[Anssi Kääriäinen]

I believe we can get rid of the Field.creation_counter hack. In Python 3 it is possible to store the attrs in a sorted dictionary. See ​http://stackoverflow.com/questions/4459531/how-to-read-class-attributes-in-the-same-order-as-declared.