fix: import timezone from datetime for rate limit header parsing

The _parse_rate_limit_headers() function uses timezone.utc on line 82
but timezone was never imported from datetime. This would raise a
NameError the first time GitHub API rate-limit headers are parsed.

Import timezone alongside datetime to fix the missing import.

src/specify_cli/__init__.py

@@ -53,7 +53,7 @@ from typer.core import TyperGroup
 import readchar
 import ssl
 import truststore
-from datetime import datetime
+from datetime import datetime, timezone
 
 ssl_context = truststore.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
 client = httpx.Client(verify=ssl_context)