Cancel workflow if another commit is pushed to the PR

[peterfpeterson]

According to the github docs one can cancel a workflow when another push is made to the same branch. This introduces that behavior.

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent