Allow .yml as YAML file extension

[tbrlpld]

Issue Summary

When defining template context files (the YAML) files, these are only recognised when they use the full .yaml extension.

When using the shorter .yml extension the template is not picking up the context file.

This can be very confusing.

Steps to Reproduce

1. Setup a project with django-patter-library enable
2. Create a template test.html in one of the directories observed by django-pattern-library with the content

{{ test_var }}

3. Create a context file test.yaml next to the template with the content

context:
  test_var: Test content

4. Open the template in the patter library and observe that you see "Test content"
5. Rename the context file mv test.yaml test.yml
6. Open the template in the patter library and observe that you see nothing. The context is not rendered anymore.

[thibaudcolas]

👍 I like the idea.

It seems this would be pretty simple to do here: https://github.com/torchbox/django-pattern-library/blob/main/pattern_library/utils.py#L148. My preference would be for us to attempt to load the .yml file if .yaml doesn’t load, so projects can mix and match extensions.

PR welcome!