Change changes-workflow to run on PR

[dvora-h]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

Please provide a description of the change here.

[codecov-commenter]

Codecov Report

Merging #1983 (0d660cc) into master (857d839) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1983      +/-   ##
==========================================
- Coverage   93.04%   93.03%   -0.02%     
==========================================
  Files          77       77              
  Lines       16603    16603              
==========================================
- Hits        15449    15447       -2     
- Misses       1154     1156       +2     

Impacted Files	Coverage Δ
tests/test_cluster.py	98.16% <0.00%> (-0.13%)	⬇️

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[chayim]

Cool. Didn't know about this.