Fix: Remove upper constraint for phpunit/phpunit

[localheinz]

This PR

• removes the upper version constraint for phpunit/phpunit

Follows #95.

[codecov]

Codecov Report

Merging #96 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master    #96   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       74     74           
=======================================
  Files             7      7           
  Lines           275    275           
=======================================
  Hits            275    275

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