Re-import AttrsInstance in attrs ns (#994)

changelog.d/987.change.rst

@@ -0,0 +1 @@
+Added missing re-import of ``attr.AttrsInstance`` to the ``attrs`` namespace.

src/attrs/__init__.pyi

@@ -23,6 +23,7 @@ from attr import __version_info__ as __version_info__
 from attr import _FilterType
 from attr import assoc as assoc
 from attr import Attribute as Attribute
+from attr import AttrsInstance as AttrsInstance
 from attr import cmp_using as cmp_using
 from attr import converters as converters
 from attr import define as define