Add strict/complete typing

[bluetech]

The current typing is very imcomplete. To make the plugin more maintainable and understandable, add typing to all of the currently untyped code and apply stricter mypy config.

Prerequisite tasks:

• execnet typing (done, needs an execnet release + bump execnet requirement)
• bump min pytest requirement from 6.2 to 7.0 -- while 6.2 is typed it is missing a lot of stuff, 7.0 is the first version with mostly complete typing