Improve `django.template`

[noelleleigh]

• django.template.loader.get_template returns django.template.base.Template.
• django.template.loader.select_template returns django.template.base.Template.
• django.template.utils.EngineHandler.__iter__ returns an iterator of django.template.backends.base.BaseEngine.
• django.template.utils.get_app_template_dirs returns a tuple of pathlib.Path.