Add stubs for greenlet (#10463)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stubs/greenlet/@tests/stubtest_allowlist.txt

@@ -0,0 +1,7 @@
+# Error: is not present in stub
+# =============================
+# this module only contains C code and exports no Python code, so it's better
+# if we pretend it doesn't exist
+greenlet.platform
+# the tests should not be part of the modules
+greenlet.tests

stubs/greenlet/@tests/test_cases/check_greenlet.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+from typing import Optional
+from typing_extensions import assert_type
+
+import greenlet
+
+g = greenlet.greenlet()
+h = greenlet.greenlet()
+assert_type(g.parent, Optional[greenlet.greenlet])
+g.parent = h
+
+# Although "parent" sometimes can be None at runtime,
+# it's always illegal for it to be set to None
+g.parent = None  # type: ignore