Remove use of deprecated imp module in job system.

[mblayman]

Hello, django-extensions devs! Thanks for a great package. I use django-extensions all the time.

This PR removes the following Python deprecation:

venv/lib/python3.9/site-packages/django_extensions/management/jobs.py:4
  /Users/matt/projects/homeschool/venv/lib/python3.9/site-packages/django_extensions/management/jobs.py:4: 
  DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation
  for alternative uses
    from imp import find_module

Since Python 3.3, import_module imports the parent modules. I believe this mimics the behavior of the existing code that is using find_module. https://docs.python.org/3/library/importlib.html#importlib.import_module

import_module will also raise an ImportError if the module name is not found. This matches the expected error interface of find_job_module.

I took the liberty of adding a type annotation and docstring while I was at it.

I did not write any additional tests because this portion of the code has decent test coverage, and I saw that the existing tests cover find_job_module.