Update setup.cfg so psychtoolbox == 3.0.16

[isolver]

Latest release of psychtoolbox causes a wrong argument type error. Using 3.0.16 works for now.

[codecov]

Codecov Report

Merging #3694 (937beb6) into release (49cf52e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           release    #3694   +/-   ##
========================================
  Coverage    43.24%   43.24%           
========================================
  Files          267      267           
  Lines        52798    52798           
  Branches      9108     9108           
========================================
  Hits         22831    22831           
  Misses       27647    27647           
  Partials      2320     2320           

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[coveralls]

Coverage remained the same at 47.076% when pulling 937beb6 on isolver:hotfix-force-ptb-3.0.16 into 49cf52e on psychopy:release.

[isolver]

merging to try and help self install problems for now.