TST: Use ci-watson

[pllim]

See spacetelescope/hstcal#71 for example usage. This is more of reducing maintenance burden going forward, so no terribly urgent. Follow up of #48. Address part of spacetelescope/ci_watson#8.

Might be a good idea to wait for spacetelescope/drizzlepac#174 to pave the way first.

[SaOgaz]

I suspect this will fix these pytest warnings:

tests/helpers/mark.py:12
/Users/ogaz/stistools/tests/helpers/mark.py:12: PytestDeprecationWarning: the pytest.config global is deprecated. Please use request.config or pytest_configure (if you're a pytest plugin) instead.
not pytest.config.getoption("--runslow"),

tests/helpers/mark.py:22
/Users/ogaz/stistools/tests/helpers/mark.py:22: PytestDeprecationWarning: the pytest.config global is deprecated. Please use request.config or pytest_configure (if you're a pytest plugin) instead.
not pytest.config.getoption('--bigdata'),