Improve error message for eventlet/gevent monkey patching

[A5rocks]

Fixes #3087

Unfortunately I am unable to check this because I only have Linux and Windows available. This only happens on MacOS.

[CoolCat467]

Also unable to test myself, but looks good as far as I can tell

[A5rocks]

@yurivict would you mind testing this to make sure the better error message appears?

[graingert]

I think the error message could be improved by saying which patchers are present, I'll suggest a change

my review should not be blocking

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.63%. Comparing base (f724be5) to head (5e01bf0).
Report is 26 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3140    +/-   ##
========================================
  Coverage   99.62%   99.63%            
========================================
  Files         122      122            
  Lines       18360    18765   +405     
  Branches     1222     1280    +58     
========================================
+ Hits        18291    18696   +405     
  Misses         47       47            
  Partials       22       22            

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	99.02% <ø> (ø)

... and 3 files with indirect coverage changes

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