feat: set label descriptions in repo_checks

repo_checks currently normalizes label names and colors. With this
commit, it will also normalize the label description.

In interest of minimizing the effect of running the EnsureLabels check:
For labels that generally have already-established descriptions, we use
those. For labels that don't, we just use !!null for now, with the plan
of adding descriptions in the near future.

Part of:
https://openedx.atlassian.net/wiki/spaces/COMM/pages/3695214629/openedx+org-wide+labels#2.-Proposal%3A-List-of-starter-labels

labels.yml

@@ -3,15 +3,20 @@
 
 - name: ":hammer_and_wrench: maintenance"
   color: "169509"  # grassy green
+  description: !!null
 
 - name: "waiting on author"
   color: "bfd6f6"  # baby blue
+  description: "The PR or issue is waiting on a response from the author"
 
 - name: "closed-inactivity"
   color: "dbcd00"  # gold
+  description: "Indicates a PR has been closed because the author has been inactive for a long period of time."
 
 - name: "needs test run"
   color: "f5424b"  # crimson red
+  description: !!null
 
 - name: "good first issue :tada:"
   color: "43dd35"  # lime green
+  description: !!null

migrate/repo_checks.py

@@ -415,6 +415,7 @@ class EnsureLabels(Check):
     # Each item should be a dict with the fields:
     #  name: str
     #  color: str (rrggbb hex string)
+    #  description: str
     labels: list
 
     with open(LABELS_YAML_PATH) as labels_yaml:
@@ -441,6 +442,7 @@ def check(self):
             self._simplify_label(label.name): {
                 "color": label.color,
                 "name": label.name,
+                "description": label.description,
             }
             for label in existing_labels_from_api
         }
@@ -498,6 +500,7 @@ def fix(self, dry_run=False):
                         self.repo_name,
                         name=current_label["name"],
                         color=new_label["color"],
+                        description=new_label["description"],
                         new_name=new_label["name"],
                     )
                 except HTTP4xxClientError as e: