Don't use autospec in mock_sheet

1b5d844 fixed a typo, however only this
typo actually made mock_sheet work due to the intended
GenericResourceSheet spec not being used.

This now removes the autospec again -> FIXME - joka will have a look
sometime later.

src/adhocracy/adhocracy/testing.py

@@ -114,10 +114,11 @@ def node() -> colander.MappingSchema:
 @fixture()
 def mock_sheet() -> Mock:
     """Mock :class:`adhocracy.sheets.GenericResourceSheet`."""
-    from adhocracy.sheets import GenericResourceSheet
     from adhocracy.interfaces import sheet_metadata
     from adhocracy.interfaces import ISheet
-    sheet = Mock(spec=GenericResourceSheet)
+    # FIXME: Use spec=GenericResourceSheet for Mock; however this fails if the
+    # mock object is deepcopied.
+    sheet = Mock()
     sheet.meta = sheet_metadata._replace(isheet=ISheet)
     return sheet