Bump version to 0.4.2.post1

Chatto server is still at 0.4.2. Post-releases (0.4.2.postN) sort
above 0.4.2 in PEP 440 and are not pre-releases, so pip install
chattolib auto-picks them up without --pre. They also sort below any
0.4.3.dev/0.4.3a/0.4.3 release, leaving room for the next Chatto
version bump.

Replaces 0.4.2a2, which sorted below 0.4.2 and required --pre.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "chattolib"
-version = "0.4.2a2"
+version = "0.4.2.post1"
 description = "Async Python client library for the Chatto webchat Connect API"
 requires-python = ">=3.11"
 license = "MIT"