Split the release PR banner's two clauses on a period

The lead ended on the colon introducing the package list, and carried another
one mid-sentence. Two in one line read as a stutter.

Author: kdeldycke

repomatic/dep_sources.py

@@ -1216,7 +1216,7 @@ def declaration_anchor(
 
 
 UNSHIPPABLE_BANNER_LEAD = (
-    "Do not merge yet: this release would ship dependencies its users cannot install:"
+    "Do not merge yet. This release would ship dependencies its users cannot install:"
 )
 """Opening of the blocked form of the release PR's verdict.