use pathlib for join path instead of os.path.join

[danialkeimasi]

After Django 3.0, BASE_DIR in settings.py is generated by pathlib (also in this template):
quote from Django docs:

The settings.py generated by the startproject command now uses pathlib.Path instead of os.path for building filesystem paths.

BASE_DIR = Path(__file__).resolve().parent.parent

The proper way to join path with pathlib is like this:

Build paths inside the project like this: BASE_DIR / 'subdir'.

But it was used the old os.path.join
Also, The pathlib library is included in all versions of python >= 3.4. and django supported this, so there is no compatibility issue.