Fix GA behavior on pull request

[yakky]

Description

Fix current GA behavior on pull request:

• add pullrequest event
• fix towncrier check on GA checkout

References

Fix #575

Checklist

• I have read the contribution guide
• Code lint checked via inv lint
• changes file included (see docs)
• Usage documentation added in case of new features
• Tests added

[codecov]

Codecov Report

Merging #576 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #576   +/-   ##
========================================
  Coverage    93.30%   93.30%           
========================================
  Files           22       22           
  Lines         1015     1015           
  Branches       135      135           
========================================
  Hits           947      947           
  Misses          29       29           
  Partials        39       39           

Flag	Coverage Δ
#unittests	93.30% <ø> (ø)

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[coveralls]

Coverage remained the same at 96.14% when pulling 1d57f44 on bugfix/issue-575-pull-request-event into c5bb3cd on develop.