feat(python-sdk): add webhook verification and event handling (#344)

* feat(python-sdk): add webhook verification and event handling

Add webhook support to the Python SDK matching the JS SDK implementation:
- Add Webhooks class with verify() and construct_event() methods
- Implement HMAC-SHA256 signature verification with timing-safe comparison
- Add timestamp validation with configurable tolerance (default 5 minutes)
- Add comprehensive webhook event types (18 events: email, contact, domain, test)
- Add WebhookVerificationError with typed error codes
- Export webhook constants (headers) and types

* fix(python-sdk): harden webhook parsing and typing

Normalize invalid UTF-8 webhook payloads to INVALID_BODY errors so verify() safely returns false, and narrow base email webhook event types to avoid discriminated-union overlap. Add regression tests for both paths.

* chore(python-sdk): bump package version to 0.2.9

* feat(python-sdk): add local webhook test example project

Add a runnable Flask receiver and signed webhook sender under packages/python-sdk/example, and link it from the Python SDK README for local verification.

---------

Co-authored-by: Claude <noreply@anthropic.com>

packages/python-sdk/usesend/__init__.py

@@ -3,6 +3,26 @@
 from .usesend import UseSend, UseSendHTTPError
 from .domains import Domains  # type: ignore
 from .campaigns import Campaigns  # type: ignore
+from .webhooks import (
+    Webhooks,
+    WebhookVerificationError,
+    WEBHOOK_SIGNATURE_HEADER,
+    WEBHOOK_TIMESTAMP_HEADER,
+    WEBHOOK_EVENT_HEADER,
+    WEBHOOK_CALL_HEADER,
+)
 from . import types
 
-__all__ = ["UseSend", "UseSendHTTPError", "types", "Domains", "Campaigns"]
+__all__ = [
+    "UseSend",
+    "UseSendHTTPError",
+    "types",
+    "Domains",
+    "Campaigns",
+    "Webhooks",
+    "WebhookVerificationError",
+    "WEBHOOK_SIGNATURE_HEADER",
+    "WEBHOOK_TIMESTAMP_HEADER",
+    "WEBHOOK_EVENT_HEADER",
+    "WEBHOOK_CALL_HEADER",
+]