Don't rely on mock in the plugin

setuptools/tests/fixtures.py

-try:
-    from unittest import mock
-except ImportError:
-    import mock
 import pytest
 
 from . import contexts
 
 
 @pytest.yield_fixture
-def user_override():
+def user_override(monkeypatch):
     """
     Override site.USER_BASE and site.USER_SITE with temporary directories in
     a context.
     """
     with contexts.tempdir() as user_base:
-        with mock.patch('site.USER_BASE', user_base):
+            monkeypatch.setattr('site.USER_BASE', user_base)
             with contexts.tempdir() as user_site:
-                with mock.patch('site.USER_SITE', user_site):
+                    monkeypatch.setattr('site.USER_SITE', user_site)
                     with contexts.save_user_site_setting():
                         yield