Remove unittest.main() in test modules.

This construct isn't used anymore these days.

Running python tests/test_foo.py puts the tests/ directory on
PYTHONPATH, which isn't representative of how we run tests.

Use python -m unittest tests/test_foo.py instead.

tests/test_hf_api.py

@@ -102,7 +102,3 @@ class HfFolderTest(unittest.TestCase):
         # ^^ not an error, we test that the
         # second call does not fail.
         self.assertEqual(HfFolder.get_token(), None)
-
-
-if __name__ == "__main__":
-    unittest.main()