fix: Simplify the actor thread's name

[jodal]

Since Python 3.10, the thread target's name has been automatically added to the thread name if the name wasn't explicitly provided, causing thread names like ActorClassName-N (_actor_loop).

This explicitly sets the thread name, using our own counter to differentiate multiple actor threads of the same type. This brings us back to how thread names was set by Pykka pre Python 3.10, with names like ActorClassName-N.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.32%. Comparing base (d9c4a3c) to head (9dd731e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #247   +/-   ##
=======================================
  Coverage   94.31%   94.32%           
=======================================
  Files          14       14           
  Lines         580      581    +1     
  Branches       50       50           
=======================================
+ Hits          547      548    +1     
  Misses         29       29           
  Partials        4        4           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.