Fix metaclass conflict

This fixes the regression when #88 combined with #89: mypy-1.0 started
producing the error:

```
Metaclass conflict: the metaclass of a derived class must be a
(non-strict) subclass of the metaclasses of all its bases
```

src/mypy_zope/plugin.py

@@ -719,13 +719,8 @@ def _apply_interface(self, impl: TypeInfo, iface: TypeInfo) -> None:
         # there is a decorator for the class that will create a "type promotion",
         # but ensure this only gets applied a single time per interface.
         promote = Instance(iface, [])
-        if not any(promote in ti._promote for ti in impl.mro):
-            faketi = TypeInfo(SymbolTable(), iface.defn, iface.module_name)
-            faketi._promote = [promote]
-            faketi.metaclass_type = iface.metaclass_type
-            # Insert the TypeInfo before the builtins.object that's at the end.
-            assert impl.mro[-1].fullname == 'builtins.object'
-            impl.mro.insert(len(impl.mro) - 1, faketi)
+        if promote not in impl._promote:
+            impl._promote.append(promote)
 
 
 def plugin(version: str) -> PyType[Plugin]:

tests/test_mro_calculation.py

@@ -16,53 +16,3 @@ def mypy_cache_dir(tmp_path_factory):
     tdir = tmp_path_factory.mktemp('.mypy_cahe')
     print("Setup cache", str(tdir))
     return str(tdir)
-
-
-def test_mro_computation_in_forward_reference_to_implementer(mypy_cache_dir: str) -> None:
-    sample_name = "forward_reference_to_implementer"
-
-    opts = options.Options()
-    opts.show_traceback = True
-    opts.namespace_packages = True
-    opts.cache_dir = mypy_cache_dir
-    opts.plugins = ['mypy_zope:plugin']
-    # Config file is needed to load plugins, it doesn't not exist and is not
-    # supposed to.
-    opts.config_file = 'not_existing_config.ini'
-
-    samplefile = os.path.join(SAMPLES_DIR, f"{sample_name}.py")
-    base_dir = os.path.dirname(samplefile)
-    with open(samplefile) as f:
-        source = BuildSource(
-            None,
-            module=sample_name,
-            text=f.read(),
-            base_dir=base_dir,
-        )
-    result = build.build(sources=[source], options=opts)
-    assert result.errors == []
-
-    # Result.graph is a map from module name to state objects.
-    state: State = result.graph[sample_name]
-
-    # Find Mypy's representation of the Protocol class.
-    node: Optional[SymbolTableNode] = None
-    for fullname, symbol_table_node, _type_info in state.tree.local_definitions():
-        # Use startswith(...) rather than a direct comparison
-        # because the typename includes a line number at the end
-        if fullname.startswith(f"{sample_name}.Protocol"):
-            node = symbol_table_node
-            break
-
-    assert node is not None, f"Failed to find `Protocol` class in mypy's state for {samplefile}"
-
-    mro: List[TypeInfo] = node.node.mro
-    # Expected: [
-    #   <TypeInfo forward_reference_to_implementer.Protocol@21>,
-    #   <TypeInfo forward_reference_to_implementer.IProtocol>,
-    #   <TypeInfo builtins.object>,
-    # ]
-    assert len(mro) == 3
-    assert mro[0].fullname.startswith(f"{sample_name}.Protocol")
-    assert mro[1].fullname == f"{sample_name}.IProtocol"
-    assert mro[2].fullname == "builtins.object"