fix(gazelle): Include YAML 'docstart' in gazelle manifest file

[dougthor42]

Update gazelle_python.yaml to include the YAML docstart string:

-- a/gazelle_python.yaml
+++ b/gazelle_python.yaml
@@ -3,6 +3,7 @@
 # To update this file, run:
 #   bazel run //:gazelle_python_manifest.update
 
+---
 manifest:
   modules_mapping:
     30fcd23745efe32ce681__mypyc: black

While technically not required, it is good practice to include. And then users don't have to exclude gazelle_python.yaml from their linters 🙃.

/cc @joshgc

[aignas]

LGTM, I think there may be some tests fail.

[dougthor42]

I think there may be some tests fail.

Yeah, caught the one I missed and then had to re-run for a flake (seems like this one just crashed out instead of being an actual failure)