Load templates from template engines’ DIRS setting

[thibaudcolas]

Description

This makes the pattern library search for templates both in DIRS directories for template engines, and in the templates subdirectory inside each installed application if using APP_DIRS as it was before.

This is necessary so the package is usable for projects that don’t use APP_DIRS.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

[bcdickinson]

Changes LGTM, thanks Thibaud. I had some very minor feedback about commenting the lines that manipulate the directory lists in the tests.

[thibaudcolas]

Thank you @bcdickinson! I’ve refactored the tests, and also fixed a compatibility issue with Django 3.2 (dir entries are now pathlib.Path instances). I made no changes to the actual implementation so will now merge without further review.