Error strings accessed differently in pytest 5.x

Until now, I've been using this approach to check error strings in the test suite.

```python
with pytest.raises(ValueError) as ve:
    microhapulator.some.operation(baddata)
assert 'expected error message' in str(ve)
```

The `str(ve)` approach no longer works with pytest 5.x. The pytest version has been pinned temporarily in the CI configuration, but this should be addressed soon.