Some typechecks fail with "Cannot determine consistent method resolution order"

[jaraco]

While working on #3399, there were several cases where mypy could not determine a consistent MRO:

src/allmydata/test/test_storage_client.py:94: error: Cannot determine consistent method resolution order (MRO) for "NativeStorageServerWithVersion"
src/allmydata/test/no_network.py:264: error: Cannot determine consistent method resolution order (MRO) for "_NoNetworkClient"
src/allmydata/test/web/test_web.py:192: error: Cannot determine consistent method resolution order (MRO) for "FakeDisplayableServer"
src/allmydata/test/web/test_web.py:258: error: Cannot determine consistent method resolution order (MRO) for "FakeClient"
src/allmydata/test/test_dirnode.py:1590: error: Cannot determine consistent method resolution order (MRO) for "FakeClient2"

This issue appears only to occur in some environments, though the factors that determine which environments are yet unclear.