The monkeypatch fixture undoes every change at the end of the test that requested it, which is why it cannot be requested by a fixture of any wider scope; doing so raises ScopeMismatch. That blocks the common wish to set an environment variable once for a whole module or session.
The supported route is the class behind the fixture. In a module- or session-scoped fixture, write with pytest.MonkeyPatch.context() as mp: mp.setenv("API_URL", "..."); yield, which applies the patch for the fixture's lifetime and reverses it on exit.
Two details matter for environment variables specifically. mp.setenv writes to os.environ, so anything that read the value at import time already has the old one; patch the module attribute instead when a constant was captured at import. And mp.delenv(name, raising=False) is the way to remove a variable that may not be set. For settings objects cached with functools.lru_cache, clear the cache in the same fixture or the patch has no visible effect.