conditionally depend on importlib_resources (#7116)

In python >= 3.9, it's available as part of the stdlib.

src/api/python/setup.py

@@ -341,7 +341,7 @@ setup(
     license='MIT License',
     keywords=['z3', 'smt', 'sat', 'prover', 'theorem'],
     packages=['z3'],
-    install_requires = ['importlib-resources'],
+    install_requires = ["importlib-resources; python_version < '3.9'"],
     include_package_data=True,
     package_data={
         'z3': [os.path.join('lib', '*'), os.path.join('include', '*.h'), os.path.join('include', 'c++', '*.h')]