Add empty constraints file to fix tox

[dosaboy]

If we want to keep the ability to have tox support constraints passed as env we need to have a
constraints.txt file even if it is empty.

[nicolasbock]

LGTM. I don't understand why the CI is falling though.

ERROR cannot deploy bundle: cannot resolve charm or bundle "mysql-router": resolving with preferred channel: Post "https://api.charmhub.io/v2/charms/refresh": net/http: TLS handshake timeout

[freyes]

something is really messed up here, not sure if it's the repo or the PR or $something_else

Initialized empty Git repository in /home/runner/work/charmed-openstack-tester/charmed-openstack-tester/.git/
git remote add origin https://github.com/openstack-charmers/charmed-openstack-tester
git config gc.auto 0
git config --get-all http.https://github.com/openstack-charmers/charmed-openstack-tester.extraheader
git -c http.extraheader="AUTHORIZATION: basic ***" fetch --tags --prune --progress --no-recurse-submodules origin +refs/heads/*:refs/remotes/origin/* +refs/pull/138/merge:refs/remotes/pull/138/merge
Error: fatal: couldn't find remote ref refs/pull/138/merge
Warning: Git fetch failed with exit code 128, back off 2.58 seconds before retry.
git -c http.extraheader="AUTHORIZATION: basic ***" fetch --tags --prune --progress --no-recurse-submodules origin +refs/heads/*:refs/remotes/origin/* +refs/pull/138/merge:refs/remotes/pull/138/merge
Error: fatal: couldn't find remote ref refs/pull/138/merge
Warning: Git fetch failed with exit code 128, back off 3.503 seconds before retry.
git -c http.extraheader="AUTHORIZATION: basic ***" fetch --tags --prune --progress --no-recurse-submodules origin +refs/heads/*:refs/remotes/origin/* +refs/pull/138/merge:refs/remotes/pull/138/merge
Error: fatal: couldn't find remote ref refs/pull/138/merge
Error: Git fetch failed with exit code: 128
Error: Exit code 1 returned from process: file name '/home/runner/runners/2.320.0/bin/Runner.PluginHost', arguments 'action "GitHub.Runner.Plugins.Repository.v1_0.CheckoutTask, Runner.Plugins"'.

[freyes]

Closing in favor of #140 that dropped the use of constraints.txt