Fixed #20766 -- Deprecated FastCGI support.
Fixed #18168 -- clarified precedence of validation

any choices set by formfield_for_choice_field are still subject
to model validation of the model field's choices attribute
Fixed #20792 -- Corrected DISALLOWED_USER_AGENTS docs.

Thanks simonb for the report.
Fixed ._meta.pk_index() virtual field failure
Fixed #18315 -- Documented QueryDict.popitem and QueryDict.pop

Thanks gcbirzan for the report.
Fixed #20679 -- Corrected CachedFilesMixin.post_process docstring.

Thanks bmispelon for the report.
Fixed #20769 -- Added "Python compatibility" section to the 1.6 release notes.
Added versionadded directive missing from b7bd708.
Fixed handling of template loader tests.

Previously, the CachedLoaderTests were never run at all.
Fixed #20625 -- Chainable Manager/QuerySet methods.

Additionally this patch solves the orthogonal problem that specialized
`QuerySet` like `ValuesQuerySet` didn't inherit from the current `QuerySet`
type. This wasn't an issue until now because we didn't officially support
custom `QuerySet` but it became necessary with the introduction of this new
feature.

Thanks aaugustin, akaariai, carljm, charettes, mjtamlyn, shaib and timgraham
for the reviews.