Stop using ctypes for traceback mutation

[A5rocks]

Fixes #405

It's kind of unnecessary since we'll rip this code out anyways once strict exception groups are the only thing supported, but for now it's simpler.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00000%. Comparing base (8a7674c) to head (e3ed59d).
Report is 6 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3203   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             124          124           
  Lines           18792        18760   -32     
  Branches         1268         1267    -1     
===============================================
- Hits            18792        18760   -32     

Files with missing lines	Coverage Δ
src/trio/_core/_concat_tb.py	100.00000% <100.00000%> (ø)

[CoolCat467]

Looks great other than a few comments that are now outdated

[jakkdl]

don't really understand the magic going on, but if tests are passing then I assume it's fine

[A5rocks]

The pypy version is simpler to understand (at least imo) as a version to compare to.

... but yeah I'm hopeful that I didn't miss anything.