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