Document ingress_per_unit lib dependencies

Document the dependency of ingress_per_unit on
the jsonschema package (which was transitively
imported by SDI with LIBPATCH 6 and below).

Provide more informative error message on the
missing jsonschema dependency for charms using
the ingress_per_unit library.

Some housekeeping on requirements.txt.

lib/charms/traefik_k8s/v0/ingress_per_unit.py

@@ -17,6 +17,8 @@
 charmcraft fetch-lib charms.traefik_k8s.v0.ingress_per_unit
 ```
 
+Add the `jsonschema` dependency to the `requirements.txt` of your charm.
+
 ```yaml
 requires:
     ingress:
@@ -50,7 +52,14 @@ def _handle_ingress_per_unit(self, event):
 import warnings
 from typing import Dict, Optional, Tuple, TypeVar, Union
 
-import jsonschema
+try:
+    import jsonschema
+except ModuleNotFoundError:
+    raise Exception(
+        "The ingress_per_unit library needs the jsonschema package as dependency; "
+        "add 'jsonschema' to the 'requirements.txt' of your charm."
+    )
+
 import yaml
 from ops.charm import CharmBase, RelationBrokenEvent, RelationEvent
 from ops.framework import EventSource, Object, ObjectEvents
@@ -65,14 +74,14 @@ def _handle_ingress_per_unit(self, event):
 )
 
 # The unique Charmhub library identifier, never change it
-LIBID = "7ef06111da2945ed84f4f5d4eb5b353a"  # can't register a library until the charm is in the store 9_9
+LIBID = "7ef06111da2945ed84f4f5d4eb5b353a"
 
 # Increment this major API version when introducing breaking changes
 LIBAPI = 0
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 7
+LIBPATCH = 8
 
 log = logging.getLogger(__name__)
 

requirements.txt

@@ -2,4 +2,7 @@ ops == 1.3.0  # 1.4.0 breaks SDI-based tests
 deepmerge
 lightkube >= 0.8.1
 lightkube-models >= 1.22.0.4
-serialized-data-interface>=0.4.0
+# The following is needed by lib/charms/traefik_k8s/v0/ingress_per_unit.py
+jsonschema
+# The following is needed by lib/charms/traefik_k8s/v0/ingress.py
+serialized-data-interface==0.4.0  # Pin to avoid incompatible changes until dropped