Build COPR builds for maint-1.3

This change will cause automatic building of COPR builds after
committing to `maint-1.3` branch. The `maint-1.3` branch isn't the
project default branch. Without specifying the branch, Packit builds
after committing only to the project's default branch, which is the
`main` branch at this moment. The `maint-1.3` branch used to be default
branch, but after we switched the default branch to main, the COPR
builds on maint-1.3 stopped to be built.

Related documentation:
https://packit.dev/docs/configuration/upstream/copr_build

.packit.yaml

@@ -27,3 +27,5 @@ jobs:
 
 - <<: *build
   trigger: commit
+  metadata:
+    branch: maint-1.3