Fixed #12756: Improved error message when yaml module is missing.

Fixed #16502 -- Fixed a TemplateDoesNotExist error that should be an ImproperlyConfigured.

Assistance on the patch from #jambronrose.

When a BadSerializer instance is stubbed in for the yaml serializer,
make sure tests do not fail.

In order to verify the behavior of using the yaml serializer when yaml
is on the system, fake the ImportError when the serializer is being
registered.

It's now forbidden to call queryset.update(field=instance) when instance
hasn't been saved to the database ie. instance.pk is None.

Fixed #21058 -- Fixed debug view blowing up when no template is provided to the template rendering functions.

Assistance on this commit from @jambonrose.

adds fix for SingleObjectTemplateResponseMixin raising a TemplateDoesNotExist when it should have raised an ImproperlyConfigured. fixes 16502. by @ianawilson, @jambonrose

Fixed #19295 -- Documented that CachedStaticFilesStorage isn't compatible with runserver --insecure.

Fixed #20821 -- Added tooltips to Admin SelectBox widget

adds fix and test for when a template is not specified at all to render(). fixes #21058. by jambonrose and ianawilson

The Admin widget, which can be used to filter multiple selects
can sometimes be too narrow and hide information such as
user permissions. This commit adds tooltips to the select
options so that a user can hover over and see the hidden text.

Fixed #21049 -- Fixed autoreload for Python 3

Moved the get_serializer() call within the condition that checks public
serializers.  This will allow exceptions other than
SerializerDoesNotExist to be raised in order to provide the caller with
useful information, e.g when pyyaml is not installed.

The serializer definitely exists, but the dependent yaml module may not
be installed.  The register_serializer() function will catch exceptions
and will stub in a fake serializer object that will raise the exception
when the serializer is used.

This test makes sure an YAML import errors are communicated to the
caller rather than stating the serializer does not exist.

Thanks craigbruce.

Thanks Keryn Knight for the report.

Changed th system module values check to return a list.
In Python 3 it returns a dict_view which could occassionally produce
a runtime error of "dictionary changed size during iteration".

re-indented method documentation within RelatedManager

NaN, +Inf, and -Inf are no longer valid values for FloatFields.

Fixed #20920 -- Consolidated F() and Q() documentation

AdminSite now has overridable site_header, site_title and index_title attributes. Changed
each admin view to pass these to the context (in a new AdminSite.each_context() method).
The intent here is to make it easier to override these things in the common case, instead of
having to override a template, which is a bigger burden.