sphinx/view_in_gitlab: Mark extension safe for parallel reading

It should fix doc CI builds as parallel build was recently enabled in jobs.

swh/docs/sphinx/view_in_gitlab.py

@@ -32,3 +32,4 @@ def html_page_context(app, pagename, templatename, context, doctree):
 
 def setup(app):
     app.connect("html-page-context", html_page_context)
+    return {"parallel_read_safe": True}