How do I reference my settings file from running my unit tests?

I'm using Django and Python 3.7. I have different database configurations per environment, so I wanted to create settings that only import the database config depending on the environment. I have this directory ...

mainpage_project/settings

and within it are these files. "base.py" contains settings that I wish to be included regardless of the environment taht is loaded ...

(venv) localhost:mainpage_project davea$ ls mainpage_project/settings
__pycache__  base.py  dev.py  prod.py  test.py

I have this in my mainpage_project/settings/test.py file ...

from mainpage.base import *

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': 'testdatabase'
    },
}

But when I go to run my test file, I get the error complaining that it cannot execute the first line of the import, "from mainpage.base import *" ...

(venv) localhost:mainpage_project davea$ cd /Users/davea/Documents/workspace/mainpage_project; source ./venv/bin/activate; python manage.py test
Traceback (most recent call last):
  File "manage.py", line 21, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/core/management/commands/test.py", line 26, in run_from_argv
    super().run_from_argv(argv)
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/core/management/base.py", line 308, in run_from_argv
    parser = self.create_parser(argv[0], argv[1])
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/core/management/base.py", line 282, in create_parser
    self.add_arguments(parser)
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/core/management/commands/test.py", line 47, in add_arguments
    test_runner_class = get_runner(settings, self.test_runner)
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/test/utils.py", line 301, in get_runner
    test_runner_class = test_runner_class or settings.TEST_RUNNER
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/conf/__init__.py", line 57, in __getattr__
    self._setup(name)
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/conf/__init__.py", line 44, in _setup
    self._wrapped = Settings(settings_module)
  File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/django/conf/__init__.py", line 107, in __init__
    mod = importlib.import_module(self.SETTINGS_MODULE)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/davea/Documents/workspace/mainpage_project/mainpage_project/settings/test.py", line 1, in <module>
    from mainpage.base import *
ModuleNotFoundError: No module named 'mainpage.base'

How do I properly reference my settings?