Bump version to 0.4.0

Releases the unreleased work since v0.3.0:
- Liberal MQTT URI normalization + extract_mqtt_broker_uris helper
  (Postel's Law for /notifiers responses)
- VenClient.get_mqtt_broker_uris() convenience method
- pendulum.DateTime propagation locked in by tests
- CONTRIBUTING.md and a Time-and-timezones README section

Refresh stale User-Agent examples in README (0.2.1 → 0.4.0).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dcj

README.md

@@ -70,7 +70,7 @@ ven_token = base64.b64encode(b"ven_client:999").decode()
 Clients send a composed User-Agent header for server-side log identification:
 
 ```
-python-oa3-client/0.2.1 openadr3/0.3.0 (node=a1b2c3d4e5f6)
+python-oa3-client/0.4.0 openadr3/0.3.0 (node=a1b2c3d4e5f6)
 ```
 
 Add your own identifier with the `user_agent` parameter:
@@ -81,7 +81,7 @@ ven = VenClient(
     token=token,
     user_agent="my-thermostat/1.0 (contact@example.com)",
 )
-# UA: python-oa3-client/0.2.1 openadr3/0.3.0 (node=...) my-thermostat/1.0 (contact@example.com)
+# UA: python-oa3-client/0.4.0 openadr3/0.3.0 (node=...) my-thermostat/1.0 (contact@example.com)
 ```
 
 ## mDNS/DNS-SD Discovery

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "python-oa3-client"
-version = "0.3.0"
+version = "0.4.0"
 description = "OpenADR 3 companion client with VEN registration, MQTT notifications, and lifecycle management"
 readme = "README.md"
 requires-python = ">=3.10"