pytester: use monkeypatch.chdir() for dir changing

The current method as the following problem, described by Sadra
Barikbin:

The tests that request both `pytester` and `monkeypatch` and use
`monkeypatch.chdir` without context, relying on `monkeypatch`'s teardown
to restore cwd. This doesn't work because the following sequence of
actions take place:

- `monkeypatch` is set up.
- `pytester` is set up. It saves the original cwd and changes it to a
  new one dedicated to the test function.
- Test function calls `monkeypatch.chdir()` without context.
  `monkeypatch` saves cwd, which is not the original one, before
  changing it.
- `pytester` is torn down. It restores the cwd to the original one.
- `monkeypatch` is torn down. It restores cwd to what it has saved.

The solution here is to have pytester use `monkeypatch.chdir()` itself,
then everything is handled correctly.

testing/test_pathlib.py

@@ -236,15 +236,15 @@ class TestImportPath:
         name = "pointsback123"
         p = tmp_path.joinpath(name + ".py")
         p.touch()
-        for ending in (".pyc", ".pyo"):
-            mod = ModuleType(name)
-            pseudopath = tmp_path.joinpath(name + ending)
-            pseudopath.touch()
-            mod.__file__ = str(pseudopath)
-            monkeypatch.setitem(sys.modules, name, mod)
-            newmod = import_path(p, root=tmp_path)
-            assert mod == newmod
-        monkeypatch.undo()
+        with monkeypatch.context() as mp:
+            for ending in (".pyc", ".pyo"):
+                mod = ModuleType(name)
+                pseudopath = tmp_path.joinpath(name + ending)
+                pseudopath.touch()
+                mod.__file__ = str(pseudopath)
+                mp.setitem(sys.modules, name, mod)
+                newmod = import_path(p, root=tmp_path)
+                assert mod == newmod
         mod = ModuleType(name)
         pseudopath = tmp_path.joinpath(name + "123.py")
         pseudopath.touch()