Add constraints file(s) for testing environments.

constraints/README.md

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+# Constraints
+
+This directory holds the pip constraints used by the different testing
+environments used by Charmed OpenStack.
+
+It's worth emphasize that these constraints are used for testing environments,
+that means used in tox.ini files, but never to build charms.
+
+## Naming scheme
+
+The constraints files follow the following nomenclature:
+
+    constraints-$RELEASE.txt
+
+`$RELEASE` represents the OpenStack release, with the special exception of
+`master` that's used to apply constraints to the `master` and `main` branches.
+
+For example for the case of OpenStack 2023.1 (Antelope), the constraints file
+would be named `constraints-2023.1.txt`.

constraints/constraints-2024.1.txt

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+# NOTE: this constraints file can be (and will be) consumed by downstream.
+#
+# Known consumers:
+# * zosci-config: job definitions that declare what juju version (snap channel)
+#   is used in tandem with this constraints file to lockdown python-libjuju
+#   version.
+# * zaza-openstack-tests
+#
+juju>=3.1.0,<3.2.0

constraints/constraints-master.txt

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+# NOTE: this constraints file can be (and will be) consumed by downstream.
+#
+# Known consumers:
+# * zosci-config: job definitions that declare what juju version (snap channel)
+#   is used in tandem with this constraints file to lockdown python-libjuju
+#   version.
+# * zaza-openstack-tests
+#
+juju>=3.1.0,<3.2.0