[1.5.x] Fixed #19539 -- Updated custom model fields example for Python 3.

Thanks astorije@ for the report.

Backport of b9fceadfd4 from master.
[1.5.x] Fixed #14006 -- Documented that Field's 'description' attribute is interpolated with field.__dict__

Thanks abeld for the suggestion.

Backport of 2d96e660f9 from master.
[1.5.x] Fixed #20711 -- Fixed broken link in timesince.py docstring

Backport of d63327d843 from master.
[1.5.x] Fixed #12346 -- Added a note on how to validate InlineFormSets.

Thanks johnsmith for the suggestion.

Backport of 181f63c22d from master
[1.5.x] Fixed #20493 -- Added a warning that objects may not be picklable across Django versions

Thanks cataliniacob for the suggestion and review.

Backport of 1084575163 from master
[1.5.x] Fixed #16965 -- Clarified Formset can_delete docs.

Thanks gregcorey@ for the suggestion.

Backport of 660c30ed95 from master
[1.5.x] Fixed #18944 -- Documented PasswordResetForm's from_email argument as a backwards incompatible change for 1.3

Thanks DrMeers for the report.

Backport of dab921751d from master
[1.5.x] Fixed #18261 -- Clarified 'project root directory'

Backport of ef3fddf26e from master
[1.5.x] Fixed #20722 -- Fixed MemcachedCache backend get_many on Python 3.

Backport of 3c736207a3 from master
[1.5.x] Fixed #19196 -- Added test/requirements

Backport of 4d92a0bd86 from master.
[1.5.x] Added a missing test requirement in the docs; refs #19196
[1.5.x] Fixed #20075 -- Fixed session test fail when using a cache other than LocMemCache.

As override_settings was used after the initialization of the session backend,
we need to use a new session backend here.

Backport of 8c1cc4b3b0 from master
[1.5.x] Fixed #20730 -- Fixed "Programmatically creating permissions" error.

Thanks glarrain for the report.

Backport of 684a606a4e from master
[1.5.x] Fixed #20053 -- Fixed `index_together` documentation

Backport of 186bff47 from master
[1.5.x] Fixed #20735 -- clarified ManyToManyField constructor doc

Backport of ac223ff6d1 from master
[1.5.x] Fixed #17528 -- Documented that add() and remove() with a many-to-many relationship do not call Model.save()

Backport of 3cdeb572d7 from master
[1.5.x] Fixed #20681 -- Prevented teardown_databases from attempting to tear down aliases

Thanks simonpercivall.

Backport of d9c580306c from master
[1.5.x] Fixed grammatical error in auth docs

Backport of 51b50c47c7 from master
[1.5.x] Fixed some markup in docs/ref/templates/api.txt

Backport of 862a04a56d from master
[1.5.x] Reworked docs for ModelForm validation.

Backport of fba6c2ede7 from master