Spawn backend table

[goodboy]

Prep for more spawning backends (such as rsyscall as per #272).

The explicit set of backends is now: ['trio', 'mp_spawn', 'mp_forkserver'] as can be set by tractor._spawn.try_set_spawn_method().

Further this patch set also,

• fixes/updates some type annotations that seem to be failing on latest mypy
• drops the tractor.testing subpkg and moves our @tractor_test definition into tests/conftest.py
• changes the ci.yml to explicitly run the tests on each spawning backend