Cookiecutter-django: Migrate to Pathlib

[webyneter]

It's time to consider migrating to Pathlib (thanks @browniebroke for raising the concern). Affected files:

• pre_gen_project.py;
• post_gen_project.py;
• test_cookiecutter_generation.py;
• manage.py;
• wsgi.py;
• merge_production_dotenvs_in_dotenv.py;
• settings.

[browniebroke]

Just noting down that we currently cannot use it static-related settings due to an issue in Whitenoise.

[yunti]

@browniebroke @webyneter Is this a change you still wanted to make in the project as Whitenoise has resolved the issue now? Did you want to drop django-environ altogether as there looks to be functionality used in the project that wouldn't be covered by pathlib, but may not be necessary.

[browniebroke]

Would be nive to have it yes, but I don't think we want to drop django-environ.