Avoid running duplicated tests on PRs

[juhoinkinen]

The PR checks list is a bit cluttered by 6 unit test runs: tests for both push and pull_request events on 3 Python versions, while testing on both events is (mostly) useless as they are (nearly) identical cases.

On the pull_request event the tests are run on "as the PR was merged", so it is more meaningful case to test, and is needed also as the push event is not available from forks.

Running tests on all push events, however, is not so useful. On pushes the workflow is usually needed to be run only when the target is master branch (typically when a PR is merged or a release is made).

This changes the push event trigger to exclude other branches than master. Also adds workflow_dispatch trigger, which allows manually running the workflow for any branch, which may sometimes be wanted for branches without PR.

(Based on this discussion, also to follow in future for possible improvements.)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Merging #574 (88f4db8) into master (d0f84ae) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #574   +/-   ##
=======================================
  Coverage   99.47%   99.47%           
=======================================
  Files          84       84           
  Lines        5568     5568           
=======================================
  Hits         5539     5539           
  Misses         29       29           

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[osma]

This is great!