feat: add badges to README and update type hints in tests

Author: bjarn

README.md

@@ -1,5 +1,11 @@
 # Lettermint Python SDK
 
+[![PyPI Version](https://img.shields.io/pypi/v/lettermint?style=flat-square)](https://pypi.org/project/lettermint/)
+[![PyPI Downloads](https://img.shields.io/pypi/dm/lettermint?style=flat-square)](https://pypi.org/project/lettermint/)
+[![Python Version](https://img.shields.io/pypi/pyversions/lettermint?style=flat-square)](https://pypi.org/project/lettermint/)
+[![GitHub Tests](https://img.shields.io/github/actions/workflow/status/lettermint/lettermint-python/ci.yml?branch=main&label=tests&style=flat-square)](https://github.com/lettermint/lettermint-python/actions?query=workflow%3ACI+branch%3Amain)
+[![License](https://img.shields.io/github/license/lettermint/lettermint-python?style=flat-square)](https://github.com/lettermint/lettermint-python/blob/main/LICENSE)
+
 Official Python SDK for the [Lettermint](https://lettermint.co) email API.
 
 ## Installation

tests/test_webhook.py

@@ -4,6 +4,7 @@
 import hmac
 import json
 import time
+from typing import Optional, Tuple
 
 import pytest
 
@@ -17,8 +18,8 @@
 
 
 def generate_valid_signature(
-    payload: str, secret: str, timestamp: int | None = None
-) -> tuple[str, int]:
+    payload: str, secret: str, timestamp: Optional[int] = None
+) -> Tuple[str, int]:
     """Generate a valid signature for testing."""
     ts = timestamp or int(time.time())
     signed_content = f"{ts}.{payload}"