Allow GitHub Actions workflow to run on pull requests from forked repos

[samkit-jain]

I noticed in #278 that the checks were stuck at "Waiting for status to be reported". This was happening because in our workflow file we had set it to run on all push commits and apparently, that is not enough for the workflow to run on forked repos. This PR attempts to solve that by adding a pull_request_target event trigger so that the workflow is run on PRs raised from forked repos. More information can be found here and here.

[samkit-jain]

The check is failing because of the newer black version. There are 2 options:

• Pin the black version so that the checks pass and since merging in stable, no conflicts when merging stable onto develop
• Reformat per the latest black version and change the base to develop

[jsvine]

Thanks for this PR, @samkit-jain and for raising the question about black. I think we should pin black to a specific version. I've pushed a quick-fix to stable doing so, and will merge this PR.

[codecov]

Codecov Report

Merging #279 into stable will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           stable     #279   +/-   ##
=======================================
  Coverage   97.41%   97.41%           
=======================================
  Files          10       10           
  Lines        1160     1160           
=======================================
  Hits         1130     1130           
  Misses         30       30           

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