chore: normalize prerelease version to PEP 440

Author: aao-ipr-bot[bot]

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "adcp"
-version = "6.3.0-beta.6"
+version = "6.3.0b6"
 description = "Official Python client for the Ad Context Protocol (AdCP)"
 authors = [
     {name = "AdCP Community", email = "maintainers@adcontextprotocol.org"}