* refactored DjangoBehaviorTest so instead of a monolothic "test_config()" method;
  there are now a bunch of different test methods.  additionally, the ones with
  loops now use TestCase.subTest() to parameter things appropriately.

  *no changes to logic of the tests*

* added TestCase.subTest() stub & wrapper to make debugging subtests easier.

* added some documentation to test classes.

(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.

some tests depend on seeing it.

(reverts change from rev be9a9121d5a7)

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