Pin juju<3.0.0

libjuju's versioning is now tied to the version of juju controller they
support.

(cherry picked from commit e0e85c0)

requirements.txt

@@ -6,7 +6,9 @@ pyparsing<3.0.0  # pin for aodhclient which is held for py35
 aiounittest
 async_generator
 boto3
-juju!=2.8.3  # blacklist 2.8.3 as it appears to have a connection bug
+# - blacklist 2.8.3 as it appears to have a connection bug
+# - pin juju<3.0.0, libjuju's versioning is now tied to the version of juju controller they support.
+juju!=2.8.3,<3.0.0
 juju_wait
 # pyopenssl depends on a newer version of cryptography since 22.1.0
 # TypeError: deprecated() got an unexpected keyword argument 'name'