Fix mypy type error in FeatureFlagError.api_error method

Accept Union[int, str] to match APIError.status type.

Author: haacked

posthog/types.py

@@ -336,7 +336,7 @@ class FeatureFlagError:
     UNKNOWN_ERROR = "unknown_error"
 
     @staticmethod
-    def api_error(status: int) -> str:
+    def api_error(status: Union[int, str]) -> str:
         """Generate API error string with status code.
 
         Args: