13403: Disable assertion rewriting for external modules - add test for plugins

Author: Tusenka

src/_pytest/assertion/__init__.py

@@ -123,7 +123,6 @@ def rootpath(self):
             return os.path.abspath(os.sep)
 
 
-
 def install_importhook(config: Config) -> rewrite.AssertionRewritingHook:
     """Try to install the rewrite hook, raise SystemError if it fails."""
     config.stash[assertstate_key] = AssertionState(config, "rewrite")

testing/test_assertrewrite.py

@@ -1309,7 +1309,7 @@ def test_rootpath_base(self, pytester: Pytester, monkeypatch: MonkeyPatch) -> No
         monkeypatch.chdir(pytester.path)
         state = AssertionState(config, "rewrite")
         assert state.rootpath == str(pytester.path)
-        new_rootpath = pytester.path + "/test"
+        new_rootpath = str(pytester.path) + "/test"
         if not os.path.exists(new_rootpath):
             os.mkdir(new_rootpath)
         monkeypatch.chdir(new_rootpath)