Export MonkeyPatch as pytest.MonkeyPatch

We want to export `pytest.MonkeyPatch` for the purpose of
type-annotating the `monkeypatch` fixture. For other fixtures we export
in this way, we also make direct construction of them (e.g.
`MonkeyPatch()`) private. But unlike the others, `MonkeyPatch` is also
widely used directly already, mostly because the `monkeypatch` fixture
only works in `function` scope (issue pytest-dev#363), but also in other cases. So
making it private will be annoying and we don't offer a decent
replacement yet.

So, let's just make direct construction public & documented.

Author: bluetech

changelog/TBD.feature.rst

@@ -0,0 +1,3 @@
+It is now possible to construct a :class:`MonkeyPatch` object directly as ``pytest.MonkeyPatch()``,
+in cases when the :fixture:`monkeypatch` fixture cannot be used. Previously some users imported it
+from the private `_pytest.monkeypatch.MonkeyPatch` namespace.

src/_pytest/monkeypatch.py

@@ -111,8 +111,16 @@ def __repr__(self) -> str:
 
 @final
 class MonkeyPatch:
-    """Object returned by the ``monkeypatch`` fixture keeping a record of
-    setattr/item/env/syspath changes."""
+    """Helper to conveniently monkeypatch attributes/items/environment
+    variables/syspath.
+
+    Returned by the :fixture:`monkeypatch` fixture.
+
+    :versionchanged:: 6.2
+        Can now also be used directly as `pytest.MonkeyPatch()`, for when
+        the fixture is not available. In this case, use :meth:`context` or
+        remember to call :meth:`undo` explicitly.
+    """
 
     def __init__(self) -> None:
         self._setattr: List[Tuple[object, str, object]] = []

src/pytest/__init__.py

@@ -19,6 +19,7 @@
 from _pytest.main import Session
 from _pytest.mark import MARK_GEN as mark
 from _pytest.mark import param
+from _pytest.monkeypatch import MonkeyPatch
 from _pytest.nodes import Collector
 from _pytest.nodes import File
 from _pytest.nodes import Item