As part of merge:
* various: reverted the py26 compat fixes from rev 5e2f92012412
* test utils: stripped out "has_real_subtest" compat from rev c732a9e2a582,
  since now on py35+, which always has .subTest() method

(should fix long-standing issue 98)

* test_ext_django:

    - Simplified "stock config" setup code.  It now gets it's "sha_rounds" value
      from the django source, so we don't have to manually update it every time
      django changes their default.  This should require less maintenance across
      minor django releases.  (Should fix issue 98, and prevent recurrence)

    - Updated tests to account for quirks in how encoded hashes are handled.
      Specifically: None, "", and invalid hashes all cause subtly different
      behaviors across django versions.  tests pass against django 1.8 - 3.1.

    - split "empty hash" test out from the loop it shared with "null hash" test,
      since the two behave differently.

* tox: expanded envlist to explicitly test a bunch more django versions
  (1.8 - 3.1); and remove some needless "django 2.x + py2" tests

* passlib.apps: reformatted django CryptContext declarations;
  added one for django 2.1 (which dropped "django_bcrypt" it's default list)

* passlib.ext.django:

    - added internal "quirks" helper as central place to track
      minor edge-case changes between django versions.

    - passlib_to_django() helper now falls back to searching hasher classes
      directly, even if patch isn't installed.  this allows it to work
      for django hashers that have been removed from django's default list.

this inherits from ValueError, and has same text, so backwards compatible.

removing unittest2 also allowed removing "passlib.tests.backports"

of Python 3.5.