Fix mysql-operator charm test now that it uses Poetry for deps

[benhoyt]

The mysql-operator tests are failing since canonical/mysql-operator#192 was merged, which removes the requirements.txt file from that repo and puts dependencies in pyproject.toml. Presumably they'll be doing the same with the other db charms soon too, so we need to find a workaround. Example failure:

> sed -i -e "/^ops[ ><=]/d" -e "/canonical\/operator/d" -e "/#egg=ops/d" requirements.txt
> echo -e "\ngit+$GITHUB_SERVER_URL/$GITHUB_REPOSITORY@$GITHUB_SHA#egg=ops" >> requirements.txt
> shell: /usr/bin/bash -e {0}
sed: can't read requirements.txt: No such file or directory

[benhoyt]

mysql-k8s-operator just started failing for the same reason.