bump to php 7.3 and run composer update

[julienbornstein]

• remove support for php 7.1
• update libraries

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.52%. Comparing base (180dd3c) to head (a149a79).

Additional details and impacted files

@@             Coverage Diff              @@
##             master       #3      +/-   ##
============================================
+ Coverage     82.17%   82.52%   +0.34%     
+ Complexity       64       50      -14     
============================================
  Files             2        2              
  Lines           101      103       +2     
============================================
+ Hits             83       85       +2     
  Misses           18       18              

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