helpers: refactor user_or_none to keep things simple and make sure we only cache simple values - no SA objects This could potentially improve "unbounded memory usage" ... or least clarify which objects are kept alive.

pytest >= 4.6.6, < 4.7
pytest-sugar >= 0.9.2, < 0.10
pytest-benchmark >= 3.2.2, < 3.3
pytest-localserver >= 0.5.0, < 0.6
mock >= 3.0.0, < 3.1
Sphinx >= 1.8.0, < 1.9
WebTest >= 2.0.3, < 2.1
isort == 4.3.21