Adapt to the evolving CI SaaS landscape

[exarkun]

• xcode 12.3.0 was end-of-lifed August 2nd so that kind of job cannot run anymore
• circleci windows image upgrade from Python 3.9 to Python 3.10 so source pages in coverage data changed, breaking the reporter (and there is no Python 3.9 Windows job anymore)

[codecov[bot]]

Codecov Report

Merging #420 (fdc9836) into main (129fdf1) will increase coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #420      +/-   ##
==========================================
+ Coverage   96.19%   96.25%   +0.05%     
==========================================
  Files          63       63              
  Lines        7525     7525              
  Branches     1012     1012              
==========================================
+ Hits         7239     7243       +4     
+ Misses        211      209       -2     
+ Partials       75       73       -2     

Impacted Files	Coverage Δ
src/_zkapauthorizer/tahoe.py	94.09% <0.00%> (+1.47%)	:arrow_up:

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