ci: don't run tests when only markdown files are changed on a push

[kitsiosk]

Description

Currently, the Pytest CI workflow will run even when a push changes only markdown files (e.g., README.md).

With the proposed changes, a push that changes only markdown files will not trigger the workflow, thus speeding up the project's CI and saving compute resources 🌱 (see below for quantity).

Motivation

Here is an example of the behaviour described above: the commit 39520c8 changed these files: README.md and, when pushed, triggered this workflow run, which ran for ~2 CPU minutes. With the proposed changes, these 2 CPU minutes would have been saved, clearing the queue for other workflows and speeding up the CI of the project, while also saving resources in general.

Note that this is a single example out of 9 examples over the last few months; a lower estimate for the accumulated gain is 30 CPU minutes, but the actual number could be higher because our cutoff date is late May and our data go only a few months back.

This only affects the push trigger and not the pull_request trigger, so the actions related to pull requests (opening, pushing new commits on an open PR, closing) will not be affected, only the pushes to branches which do not have an open pull request.

Click here to see all the recent CI runs triggered by markdown files.

commit 39520c8 => run url
commit 0c7cfd5 => run url
commit 8fd2f8d => run url
commit 1c3ff6b => run url
commit a25da25 => run url
commit 4fbd82a => run url
commit 55c732e => run url
commit bed1b68 => run url
commit 400fc4f => run url

Test plan

No testing was performed.