hacking: force explicit import of python's mock

Since we dropped support for python 2 [1], we no longer need to use the
mock library, which existed to backport py3 functionality into py2.
Which must be done by saying::

    from unittest import mock

...because if you say::

    import mock

...you definitely will not be getting the standard library mock.
That will always import the third party mock library.

This commit adds hacking check N366 to enforce the former.

This check can be removed in the future (and we can start saying
``import mock`` again) if we manage to purge these transitive
dependencies. I'm not holding my breath.

[1]https://review.opendev.org/#/c/688593/

Change-Id: I79b0a084b8a99c54ce175a520e039e5572c5165a

cyborg/hacking/checks.py

@@ -170,3 +170,31 @@ def check_explicit_underscore_import(logical_line, filename):
     elif (translated_log.match(logical_line) or
           string_translation.match(logical_line)):
         yield(0, "M340: Found use of _() without explicit import of _ !")
+
+
+@core.flake8ext
+def import_stock_mock(logical_line):
+    """Use python's mock, not the mock library.
+
+    Since we `dropped support for python 2`__, we no longer need to use the
+    mock library, which existed to backport py3 functionality into py2.
+    Which must be done by saying::
+
+        from unittest import mock
+
+    ...because if you say::
+
+        import mock
+
+    ...you definitely will not be getting the standard library mock. That will
+    always import the third party mock library. This check can be removed in
+    the future (and we can start saying ``import mock`` again) if we manage to
+    purge these transitive dependencies.
+
+    .. __: https://review.opendev.org/#/c/688593/
+
+    N366
+    """
+    if logical_line == 'import mock':
+        yield (0, "N366: You must explicitly import python's mock: "
+                  "``from unittest import mock``")

cyborg/tests/unit/test_hacking.py

@@ -245,3 +245,13 @@ class HackingTestCase(base.TestCase):
         self.assertEqual(len(list(checks.check_explicit_underscore_import(
             "msg = _('My message')",
             "magnum/tests/other_files3.py"))), 0)
+
+    def test_import_stock_mock(self):
+        self._assert_has_errors(
+            "import mock",
+            checks.import_stock_mock, expected_errors=[(1, 0, 'N366')])
+        code = """
+                    from unittest import mock
+                    import unittest.mock
+               """
+        self._assert_has_no_errors(code, checks.import_stock_mock)

tox.ini

@@ -129,4 +129,5 @@ extension =
   M339 = checks:no_xrange
   M340 = checks:check_explicit_underscore_import
   M352 = checks:no_log_warn
+  N366 = checks:import_stock_mock
 paths = ./cyborg/hacking