[py3] fixed session file backend.
[py3] Properly encode hashlib.md5 and zip.compress args in the tests.
[py3] Fixed paginator tests.
[py3] Fixed bad unicode test
Fixed #18770 -- memcached cache backend expects byte strings as keys

Thanks thecore for the report.
Removed unneeded smart_bytes import which was introduced in f2fff84bc.
[py3] Fix some more encoding issues in tests
Run memcache tests if any cache is configured
[py3] Fixed dispatch tests
Fixed comment to match the code
[py3] Fixed slug regex

In Python 3, \w matches any Unicode character.
[py3] Fixed middleware tests.

Removed several inappropriate .encode('utf-8') calls in the process.
Used settings config for memcached tests.

By using settings config, we take advantage of custom settings like
KEY_PREFIX which allows for example to run tests in parallel.
[py3] Fixed more encoding issues in cache tests
[py3] Fixed middleware_exceptions tests.
Added import of force_unicode to utils/text.py
For backwards compatibility with stupid people like me. Refs #18772.
Fixed #18759 -- updated SECRET_KEY documentation

  Document SECRET_KEY becoming required in 1.5.

  Also expand the description slightly, and add a more prominent warning
  about the security implications of running with an exposed SECRET_KEY.
Marked up a few raw values in the settings documentation.
Fix changes introduced in fd04e7 so they don't break on Python 3.
Fixed the admin_changelist tests for python3.