Track Chatto 0.5.0a4.post1

Drop the connectrpc (and transitively pyqwest) dependency: the Connect
transport is now chattolib's own hand-rolled client (chattolib._connect),
and the generated service stubs come from a self-contained protoc plugin
(protoc-gen-chattolib) instead of protoc-gen-connect-python. Runtime
dependencies are now just protobuf and httpx, with websockets in the
[realtime] extra.

Bumped version 0.5.0a4 -> 0.5.0a4.post1.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "chattolib"
-version = "0.5.0a4"
+version = "0.5.0a4.post1"
 description = "Async Python client library for the Chatto webchat Connect API"
 requires-python = ">=3.11"
 license = "MPL-2.0 AND Apache-2.0"