Because it's unlikely that anything other than the `year` parameter will
change in the C Python API, the rest can be restricted to their logical
ranges, which improves the compile-time error checking.
While the valid ranges for the constructor parameters is the same when
expressed as either u32 or i32, since the Python API uses i32 in their
public interface, we won't have to make any changes to the signatures if
the Python behavior changes (e.g. supporting negative years) without
their API changing.
The PEP 518 way to do this is with pyproject.toml. tox doesn't support
PEP 518 yet, but we get around that by using pip install -e . as part of
the tox build until PEP 518 support arrives in tox.
This is really a test module, but the Rust convention is to put
something like this under examples/, and when it lands, we can take
advantage of "Project-based Examples for Cargo Projects" - RFC link
at https://github.com/rust-lang/rfcs/pull/2517
