Support PHP8

[koriym]

• Support PHP 8
• Drop support EOL PHP 7.2
• Switch CI from Travis to GH action
• Update strict CS
• Bump phpunit to 9.5
• Test test coverage 100%
• Add licence file
• Test it works without annotation reader

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (2.x@ff9959e). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             2.x       #24   +/-   ##
=======================================
  Coverage       ?   100.00%           
  Complexity     ?        16           
=======================================
  Files          ?         3           
  Lines          ?        45           
  Branches       ?         0           
=======================================
  Hits           ?        45           
  Misses         ?         0           
  Partials       ?         0           

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