How do I use "discover" to run tests in my "tests" directory?

Using DJango/Python 3.7. I read here -- How do I run all Python unit tests in a directory? that I could use a "discover" command to find tests in a specified directory. I want to have a "tests" folder, so I cretaed one and then ran

(venv) localhost:myproject davea$ python -m unittest discover tests
Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/__main__.py", line 18, in <module>
    main(module=None)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/main.py", line 100, in __init__
    self.parseArgs(argv)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/main.py", line 124, in parseArgs
    self._do_discovery(argv[2:])
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/main.py", line 244, in _do_discovery
    self.createTests(from_discovery=True, Loader=Loader)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/main.py", line 154, in createTests
    self.test = loader.discover(self.start, self.pattern, self.top)
  File "/usr/local/Cellar/python/3.7.2_2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/unittest/loader.py", line 344, in discover
    raise ImportError('Start directory is not importable: %r' % start_dir)
ImportError: Start directory is not importable: 'tests'

This is odd to me because I have an (empty) init file ...

(venv) localhost:myproject davea$ ls web/tests/
__init__.py  model_tests.py

What else do I need to do to get my test directory recognized?