pytest import in __init__.py makes it difficult to determine when pytest is running

[jwhitlock]

For mozilla/fx-private-relay#1809, I wanted to enable code only if we're not running pytest. There's one way to do it, and a quick way (if "pytest" in sys.modules). Because markus imports pytest early (added for #73), the quick method doesn't work.

I think I can make this work by adding a pytest plugin.

[willkg]

Fixed by #96.