Fixed #18659 -- Deprecated request.REQUEST and MergeDict

Thanks Aymeric Augustin for the suggestion.
Removed F.__deepcopy__()

The method didn't change standard __deepcopy__ in any way.
Added test for a921f06 - refs #21280.

This commit also lays the groundwork for future tests for the
makemigrations command.
Fixed #21282 -- Made HttpResponse.serialize_headers accept latin-1

Thanks Raphaël Barrois for the report and the initial patch and
Aymeric Augustin for the review.
Fixed #21284 -- Prevented KeyError swallowing in fetch_command

Thanks wildfire for the report.
Removed importing formtools tests in __init__.py

Unnecessary since introduction of DiscoveryTestRunner in 9012833af8.
Fixed #21259 -- Fixed formstools wizard for InlineFormSet.
Removed 1.6 release note text regarding password limit length.

This changed was reverted in 5d74853e156105ea02a41f4731346dbe272c2412.
Fixed #21285 -- Fixed E121,E122 pep8 warnings
Fixed #21267 -- Fixed E502 pep8 warnings
Fixed #21268 -- Fixed E303 pep8 warnings
Merge pull request #1768 from alasdairnicol/ticket_21268

Fixed #21268 -- Fixed E303 pep8 warnings
Fixed #21287 -- Fixed E123 pep8 warnings
Fixed bug causing CSRF token not to rotate on login.

Thanks Gavin McQuillan for the report.
Fixed ReST error in 1.6. release notes.
Added a warning regarding session security and subdomains.
Fixed #21212 -- Documented the reverse name for OneToOneField.

Thanks bjb at credil.org for the report.
Cleaned formatting/comments in PasswordResetFormTest
Fixed #21291 -- Ensured inactive users cannot reset their passwords

Thanks kz26 for the report and the suggested fix. Refs #19758.
Removed unused local variables in tests.