Hello, I'm reporting a bug in whcih all mutants are reported as surviving with python3.9. To illustrate, I used the example from the README.

# calculator.py
def mul(x, y):
    return x * y

# test_calculator.py
from unittest import TestCase
from calculator import mul

class CalculatorTest(TestCase):
    def test_mul(self):
        self.assertEqual(mul(2, 2), 4)

Running mut.py --target calculator --unit-test test_calculator -m gives the following output:

[0.39588 s] survived
[*] Mutation score [1.38427 s]: 0.0%
   - all: 3
   - killed: 0 (0.0%)
   - survived: 3 (100.0%)
   - incompetent: 0 (0.0%)
   - timeout: 0 (0.0%)

With [email protected], mutants are killed as expected. With [email protected] all mutants are reported as surviving.

I'm on macOS Big Sur (not an M1 if that matters).