Publish MonkeyPatch for typing

This builds on the work done in 2bcad38
to add `MonkeyPatch` to pytest's public API. This will allow users of
pytest to annotation their tests like

    def test_some_code(monkeypatch: MonkeyPatch) -> None:
        ...

src/pytest/__init__.py

@@ -21,6 +21,7 @@
 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
@@ -76,6 +77,7 @@
     "main",
     "mark",
     "Module",
+    "MonkeyPatch",
     "Package",
     "param",
     "PytestAssertRewriteWarning",