union of multi-markers is broken

```python
def test_union_broken() -> None:
    m1 = parse_marker('python_version >= "3.8" and python_version < "4.0" and sys_platform == "win32"')
    m2 = parse_marker('python_version >= "3.8" and python_version < "4.0"')

    union = m1.union(m2)
    assert str(union) == 'python_version >= "3.8" and python_version < "4.0"'
```
fails: the actual outcome is the same as the first marker.