Include mypy-zope test file verbatim

tests/isinstance_impl.py

@@ -0,0 +1,30 @@
+# Per https://github.com/Shoobx/mypy-zope/pull/92#issuecomment-1483266683
+from typing import Optional
+from zope.interface import implementer, Interface
+
+
+class IFoo(Interface):
+    ...
+
+
+@implementer(IFoo)
+class MyFoo:
+    ...
+
+
+def make_foo() -> Optional[IFoo]:
+    return MyFoo()
+
+
+x = make_foo()
+reveal_type(x)
+assert isinstance(x, MyFoo)
+
+# The code below should not be considered unreachable
+print("hello")
+
+"""
+<output>
+isinstance_impl.py:19: note: Revealed type is "Union[__main__.IFoo, None]"
+</output>
+"""