Try isolation

tests/test_modutils.py

@@ -268,19 +268,6 @@ def test_std_lib(self) -> None:
             os.path.realpath(os.path.__file__.replace(".pyc", ".py")),
         )
 
-    def test_std_lib_found_before_same_named_package_on_path(self) -> None:
-        realpath = str(resources.RESOURCE_PATH)
-        if WIN32:
-            # Escape backslashes.
-            realpath = realpath.replace("\\", "\\\\")
-        sys.path.insert(0, realpath)
-        self.addCleanup(sys.path.pop, 0)
-
-        file = modutils.file_from_modpath(["copy"])
-
-        self.assertNotIn("test", file)  # tests/testdata/python3/data/copy.py
-        self.assertTrue(any(stdlib in file for stdlib in modutils.STD_LIB_DIRS))
-
     def test_builtin(self) -> None:
         self.assertIsNone(modutils.file_from_modpath(["sys"]))
 
@@ -293,6 +280,18 @@ def test_unicode_in_package_init(self) -> None:
         modutils.file_from_modpath(["data", "unicode_package", "core"])
 
 
+class FileFromModPathTestIsolation(unittest.TestCase):
+    def test_std_lib_found_before_same_named_package_on_path(self) -> None:
+        path = str(resources.RESOURCE_PATH)
+        sys.path.insert(0, path)
+        self.addCleanup(sys.path.pop, 0)
+
+        file = modutils.file_from_modpath(["copy"])
+
+        self.assertNotIn("test", file)  # tests/testdata/python3/data/copy.py
+        self.assertTrue(any(stdlib in file for stdlib in modutils.STD_LIB_DIRS))
+
+
 class GetSourceFileTest(unittest.TestCase):
     def test(self) -> None:
         filename = _get_file_from_object(os.path)