publish heroku now deploys with Python 3.8.3

datasette/publish/heroku.py

@@ -167,7 +167,7 @@ def temporary_heroku_directory(
         if metadata_content:
             open("metadata.json", "w").write(json.dumps(metadata_content, indent=2))
 
-        open("runtime.txt", "w").write("python-3.8.0")
+        open("runtime.txt", "w").write("python-3.8.3")
 
         if branch:
             install = [