Make sure mypy treats implementations as subtype of their interface

Fixes #91

src/mypy_zope/plugin.py

@@ -3,6 +3,7 @@
 from typing import Type as PyType
 from typing import cast
 
+from mypy.typestate import type_state
 from mypy.types import (
     Type,
     Instance,
@@ -722,6 +723,13 @@ def _apply_interface(self, impl: TypeInfo, iface: TypeInfo) -> None:
         if promote not in impl._promote:
             impl._promote.append(promote)
 
+        # Remember implementation as a subtype of an interface. HACK: we are
+        # writing to a global variable here, so potentially this might be a
+        # memory leak. Needs testing with a large codebase.
+        asmpt = (Instance(impl, []), promote)
+        type_state.get_assumptions(False).append(asmpt)
+        type_state.get_assumptions(True).append(asmpt)
+
 
 def plugin(version: str) -> PyType[Plugin]:
     return ZopeInterfacePlugin