How can I mock an import error of the module foo in the unit test?

# file: bla.py
try: 
    import foo

except ImportError: 
    print("error")
    sys.exit(1)

Baseline for the unit test:

import bla
import unittest

class MyTest(unittest.TestCase):
    # what to mock (or so) here in order to raise the 'ImportError' that then raises the `sys.exit(1)`
    def test_1(self):
        with self.assertRaises(SystemExit) as err:
            importlib.reload(bla) # force reload, but fails as 'foo' is not importable
        self.assertEqual(rt.exception.code,1)