Fixed #22453 -- Corrected DoesNotExist message in docs.

Thanks Eugenio Cano-Manuel.
Fixed #22457 -- Fixed contributing guide w/o github set-up

The published commnand was not working when the user did not have
the git client set-up with the public-key. Changed the contributing
guide to clone it from https instead.
Fixed spelling errors in documentation.
Fixed #22460 -- Explicity remove constraints when dropping a related field.
Fixed flake8 warnings introduced in recent commits.
Fixed #22455 -- Documented return values for send_mail and send_mass_mail
Fixed #22434 -- Retain ordering on related sliced subqueries.

Thanks maciej.pawlisz for the report, and charettes for the review.
Fixed #22220 -- Added more examples to reverse() documention.

Thanks EvilDMP for the suggestions.
Fixed #22378 -- Updated \d to [0-9]+ in urlpatterns of docs and tests.

Thanks tomwys for the suggestion.
Revert "Fixed #22401 -- Deprecated regular expression parsing of initial SQL in favor of installing sqlparse."

This reverts commit 071c9337750b296d198cced56f3ffad0e176afb6.

This introduced a regression on MySQL and custom SQL is deprecated.
Fixed #22397 -- Issues removing M2M field with explicit through model.

Changed the migration autodetector to remove models last so that FK
and M2M fields will not be left as dangling references. Added a check
in the migration state renderer to error out in the presence of
dangling references instead of leaving them as strings. Fixed a bug
in the sqlite backend to handle the deletion of M2M fields with
"through" models properly (i.e., do nothing successfully).

Thanks to melinath for report, loic for tests and andrewgodwin and
charettes for assistance with architecture.
Use the stdlib's PBKDF2 implementation when available.

This is a bit faster than ours, which is good, because it lets you increase
the iteration counts.

This will be used on Python 3.4+, and, pending the acceptance of PEP466, on
newer Python 2.7s.
Typo fix
For another day
Include an 'extra_requires' for bcrypt
Stray paren
Fixed #22456 -- Replaced 900913 EPSG code by 3857 in some tests

3857 is now the official EPSG code for the Google spherical mercator
projection and some recent versions of GDAL do not recognize 900913
any longer.
Thanks Tim Graham for the report and initial patch.
Confirmed support for PostGIS 2.1 and GDAL 1.10

Refs #22456.
Merge pull request #2583 from alex/extras-bcrypt

Include an 'extra_requires' for bcrypt
Merge pull request #2582 from alex/stdlib-pbkdf2hmac

Use the stdlib's PBKDF2 implementation when available.