horizon recently switched from the third-party mock to unittest.mock.
This breaks heat-dashboard unit tests somehow.
We no longer support py27 and we can use the standard library
unittest.mock module. Rather than investigating the cause of
the failure, it would be nice to switch to unittest.mock.
horizon dependency is updated to 18.3.1 as unittest.mock is used
since 18.3.0 and 18.3.0 has an issue that it always requires pytest.
lower-constraints is updated to match horizon requirements.
hacking local-check-factory is disabled temporarily due to
the incompatibility with hacking 2.0 or later.
https://review.opendev.org/#/c/715690/ will recover it.