bpo-30108: Restore sys.path in test_site (#1197)

Add setUpModule() and tearDownModule() functions to test_site to
save/restore sys.path at the module level to prevent warning if the
user site directory is created, since site.addsitedir() modifies
sys.path.

Lib/test/test_site.py

@@ -27,14 +27,27 @@ if sys.flags.no_site:
 
 import site
 
-if site.ENABLE_USER_SITE and not os.path.isdir(site.USER_SITE):
-    # need to add user site directory for tests
-    try:
-        os.makedirs(site.USER_SITE)
-        site.addsitedir(site.USER_SITE)
-    except PermissionError as exc:
-        raise unittest.SkipTest('unable to create user site directory (%r): %s'
-                                % (site.USER_SITE, exc))
+
+OLD_SYS_PATH = None
+
+
+def setUpModule():
+    global OLD_SYS_PATH
+    OLD_SYS_PATH = sys.path[:]
+
+    if site.ENABLE_USER_SITE and not os.path.isdir(site.USER_SITE):
+        # need to add user site directory for tests
+        try:
+            os.makedirs(site.USER_SITE)
+            # modify sys.path: will be restored by tearDownModule()
+            site.addsitedir(site.USER_SITE)
+        except PermissionError as exc:
+            raise unittest.SkipTest('unable to create user site directory (%r): %s'
+                                    % (site.USER_SITE, exc))
+
+
+def tearDownModule():
+    sys.path[:] = OLD_SYS_PATH
 
 
 class HelperFunctionsTests(unittest.TestCase):