Let the Document class suggest the default filename for the safe PDF,
based on the provided input filename, appended with the extension
`-safe.pdf`.
Previously, this logic was copy-pasted throughout the code, which made
it difficult to maintain.
Run Mypy static checks against our tests. This brings them inline with
the rest of the codebase, and we have an extra level of certainty that
the tests (and unit tests in particular) will not significantly diverge
from the code they are testing.