Fix tomllib import for Python 3.10 compatibility

Use tomli (already a core dep) as fallback on Python <3.11 where
tomllib is not available.

Co-Authored-By: João <joao@crewai.com>

lib/crewai/tests/test_litellm_dependency_compat.py

@@ -6,9 +6,13 @@ conflict with crewAI's own requirements (openai, python-dotenv, etc.).
 """
 
 import sys
-import tomllib
 from pathlib import Path
 
+if sys.version_info >= (3, 11):
+    import tomllib
+else:
+    import tomli as tomllib
+
 import pytest
 from packaging.specifiers import SpecifierSet
 from packaging.version import Version