fix: add Python 3.9 compatibility to remaining integration test file

- Fix union type syntax in test_direct_api_integration.py
- Ensures all test files work with Python 3.9+
- Completes Python 3.9 compatibility across entire codebase

Author: jdrhyne

tests/integration/test_direct_api_integration.py

@@ -4,29 +4,31 @@
 test all Direct API methods against the live Nutrient DWS API.
 """
 
+from typing import Union, Optional
+
 import pytest
 
 from nutrient_dws import NutrientClient
 
 try:
     from . import integration_config  # type: ignore[attr-defined]
 
-    API_KEY: str | None = integration_config.API_KEY
-    BASE_URL: str | None = getattr(integration_config, "BASE_URL", None)
+    API_KEY: Optional[str] = integration_config.API_KEY
+    BASE_URL: Optional[str] = getattr(integration_config, "BASE_URL", None)
     TIMEOUT: int = getattr(integration_config, "TIMEOUT", 60)
 except ImportError:
     API_KEY = None
     BASE_URL = None
     TIMEOUT = 60
 
 
-def assert_is_pdf(file_path_or_bytes: str | bytes) -> None:
+def assert_is_pdf(file_path_or_bytes: Union[str, bytes]) -> None:
     """Assert that a file or bytes is a valid PDF.
 
     Args:
         file_path_or_bytes: Path to file or bytes content to check.
     """
-    if isinstance(file_path_or_bytes, str | bytes):
+    if isinstance(file_path_or_bytes, (str, bytes)):
         if isinstance(file_path_or_bytes, str):
             with open(file_path_or_bytes, "rb") as f:
                 content = f.read(8)