Updated TemplateResponseMixin docs to reflect new behavior after refs #23789.
Allowed Python executable names other than python in test from refs #24007.
Fixed #24014 -- Unified OGRException and GDALException

Thanks Tim Graham for the review.
Refs #24020 -- return expressions from get_group_by_cols()
Fixed #23788 (2) -- Improved gettext version checker

Thanks Tim Graham for the report and initial patch.
Removed need to update Python version support note on each new release.
Simplified handling of a default value.
Renamed get_template_from_string to from_string.

The shorter name is just as explicit and, well, shorter.
Cleaned up the django.template namespace.

Since this package is going to hold both the implementation of the Django
Template Language and the infrastructure for Multiple Template Engines,
it should be untied from the DTL as much as possible within our
backwards-compatibility policy.

Only public APIs (i.e. APIs mentioned in the documentation) were left.
Imported BaseEngine from the DEP.

i18n is left aside for now.
Added initial support for loading template engines.
Added dummy template backend.
Added jinja2 template backend.
Added Django template backend.
Added basic tests for template backends.
Removed some uses of global APIs from django.template.loader.
Passed a reference to the current engine when instanciating Template.
Isolated template tests from Django settings.
Looked up the default template engine in the list of all engines.
Supported multiple template engines in get_template and select_template.