Typing fixes

[palfrey]

Fixes #15.

Also

• Upgrades minimum Python to 3.7 and sets structlog requirement so we have typing support there.

[wimglenn]

Is there a point of type-hinting in the tests/*.py? IIUC users would only be interesting in typing of the plugin itself.

[palfrey]

Is there a point of type-hinting in the tests/*.py? IIUC users would only be interesting in typing of the plugin itself.

Main reason for that is to act as a test case for the "check this works when someone installs it" i.e. the "pip install without -e" case, which already caught errors in 3d88415. Also a little bit of personal preference for typing tests as well for better IDE support.