Fix path to requirements.txt

[hugovk]

Follow on from #431 to fix the CI.

Sample build: https://github.com/hugovk/python-fire/actions/runs/4068330039

[Borda]

wondering how that the PR #431 went 🟢

[hugovk]

Because it only ran the CLA check:

That was because the test workflow only triggered for push events, and for a push from a fork, means it would only run on the fork's CI, not this upstream:

on: [push]

It failed on #428 because it fixes the triggers:

on: [push, pull_request]

So it also runs on upstream for the PR trigger. This is good, so we can see if PRs from forks pass on the CI. This is very important for open source, where people often contribute from forks.