input: Write a python function to check if Django is installed in the virtual environment and return its version.
  assert check_django_version() is not None
  assert isinstance(check_django_version(), str)
  assert "Django" in check_django_version()
output: def check_django_version():
      import pkg_resources
      try:
          django_version = pkg_resources.get_distribution("Django").version
          return f"Django {django_version}"
      except pkg_resources.DistributionNotFound:
          return None