Alpha3

[goodboy]

3rd alpha release.
Also tossed in 3.10 to CI let's see if that works 😂

Notes for trio montly, ping @Fuyukai:

tractor saw its 4th alpha release with some interesting improvements in the core cross-actor cancellation mechanics as well as the start of a public expose of what were our internal trio helper API goodies and we officially now have optional msgspec support!

The quick feature summary:

• We switched to making windows use the trio/subprocess spawner by default which means windows users now get multi-core debugger support out of the box.
• Support for optional msgspec lib usage for those who want a faster and typed codec alternative to msgpack-python.
• We exposed a new sub-package tractor.trionics which is where we'll be stashing all our "higher level" trio related helper APIs going forward. For example checkout our new gather_contexts concurrent context manager enter-er (kinda like a concurrent version of asyncio.gather() but for context manager instances).
• Dropped Py 3.8 support as we decided to roll with a new model of supporting only the latest 2 major releases which means we also now support 3.10.
• Change the core message loop to handle task and actor-runtime cancel
  requests immediately instead of scheduling them as is done for rpc-task
  requests. Previously cancel requests were actually scheduled as rpc requests but this has changed to instead invoke a synchronous style cancellation which resulted in a massive reduction in teardown race conditions particularly in our anti-zombie-process machinery and it's interaction with the debugger system.

[goodboy]

There super naively dropped 3.10 on windows for now which I'm thinking is nbd for this release (because it likely works just fine).

[goodboy]

Ugh but mistakenly dropped msgspec testing for windows..
We know it works (due to previous runs) so not going to stress about fixing CI before this release.