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.

changelog/11315.trivial.rst

@@ -0,0 +1,3 @@
+The :fixture:`pytester` fixture now uses the :fixture:`monkeypatch` fixture to manage the current working directory.
+If you use ``pytester`` in combination with :func:`monkeypatch.undo() <pytest.MonkeyPatch.undo>`, the CWD might get restored.
+Use :func:`monkeypatch.context() <pytest.MonkeyPatch.context>` instead.