Build Python ABI3 wheels

[jetuk]

The updates to the Python extension in #73 are not ABI3 compatible. Building ABI3 wheels would reduce the CI to one build per platform rather requiring individual Python version wheels.

Currently this is blocked by the extension module using PyDate and PyDateAccess which are not part of the ABI. Removing these objects from the extension can be done by either:

1. Using chrono and the support in pyo3 for converting from Python datetime objects to chrono types, or
2. Replicating the above support but for the time crate that is currently used here. One would have to follow the same idea as for the chrono conversions in pyo3 and use the Python API directly in the extension.

[jetuk]

PyO3 documentation here: https://pyo3.rs/v0.20.2/building_and_distribution.html#py_limited_apiabi3

[jetuk]

Once #115 is merged (1) above can be done, and we should be able to produce ABI3 wheels.

[jetuk]

Turns out, it can't be done just yet because the features required in PyO3 haven't been released yet.

[jetuk]

#175