Remove all Python 2.x items

[mperrin]

Borrowing from the to-do list at astropy/astropy#6234

• Remove __future__ imports
• Make sure we remove the unicode_literals text fixture
• Search for and remove any 'six' imports and in some cases replace by the appropriate Python 3 import.
• Search for and remove any if six.PY2 blocks and also search for sys.version_info or any usage of minversion or Looseversion.
• Update CI configuration to not run tests on Python 2
• Search for comments that mention Python (either 2 or 3 or none of them) to see if anything can be updated
• Remove python2 related info from the documentation

[mperrin]

also

• remove the back ported version of lru_cache in the vendor directory.

[mperrin]

and update the documentation and setup config to enforce a minimum Python version of 3.4.