Make sure we're running on M1 MacOS

[A5rocks]

macos-latest doesn't seem to include macos-14: https://github.com/python-trio/trio/actions/runs/7865413592/job/21458313729#step:1:8

So this should make things run on M1 macs.

Reference: #2885

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.89%. Comparing base (b4c19bc) to head (811d289).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2953       +/-   ##
===========================================
- Coverage   99.63%   80.89%   -18.74%     
===========================================
  Files         117      117               
  Lines       17602    17557       -45     
  Branches     3174     1398     -1776     
===========================================
- Hits        17537    14202     -3335     
- Misses         46     3250     +3204     
- Partials       19      105       +86     

see 64 files with indirect coverage changes

[A5rocks]

Oh despite 3.8 and 3.9 having installers that work on arm, it looks like setup-python doesn't recognize that yet: actions/setup-python#808

From the issue it sounds like all that's missing is someone needs to build for 3.8 and 3.9 arm64? So I'm comfortable letting this sit until that happens.

[A5rocks]

This should be unblocked now

[A5rocks]

Actually this is no longer necessary

macos-latest will now be M1 macos