Bump version to 0.4.2a2

Pre-release publishing the realtime WebSocket client and remaining
Connect API surface additions (asset upload, roles, external identity
flows, admin services) against Chatto server 0.4.2. Pre-release
sorts below 0.4.2 in PEP 440, so pip install --pre is required to
resolve it.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "chattolib"
-version = "0.4.2"
+version = "0.4.2a2"
 description = "Async Python client library for the Chatto webchat Connect API"
 requires-python = ">=3.11"
 license = "MIT"