Fixed #21702 -- Added different bullet styles for nested lists.

Thanks moc at mocpa.com for the suggestion.
Fixed #21120 -- Added more explicit text on using validators and link to writing validators.

Thanks nicolas at niconomicon.net for the suggestion.
Fixed #21140 -- Added 'Using cached sessions' to the performance guide.

Thanks EvilDMP for the suggestion.
Fixed #20830 -- Clarified that Django uses a customized version of six.

Thanks glarrain for the suggestion.
Fixed #14028 - Added validation for clashing db_columns.

Thanks akaariai for the suggestion.
Fixed #21070 -- Added a more complex database configuration example.

Thanks joejasinski for the suggestion.
Fixed #21001 -- Added a regression test for empty string exclusion on Oracle.

The issue was present in 1.4.x but has been reported to be fixed in master.

Thanks to @timgraham for the patch.
Fixed test suite bisection on Python 3.

Thanks to @marfire for spotting this.
Removed implication that six is part of Python stdlib.
Removed extra p (topppings->toppings)