Use pytest for testing

Use pytest as your testing framework:
- Simple assert statements
- Powerful fixtures for setup/teardown
- Parametrized tests for multiple inputs
- Rich plugin ecosystem

```python
import pytest

@pytest.fixture
def user():
return User(name="Test", email="[email protected]")

def test_user_full_name(user):
assert user.full_name == "Test"

@pytest.mark.parametrize("input,expected", [
(1, 1), (2, 4), (3, 9)
])
def test_square(input, expected):
assert square(input) == expected
```