py.typed marker file not included in published wheel

[jamesdow21]

Related to #223

The changes from #224 should have resulted in the py.typed marker file being included in the published wheel (following the instructions in the setuptools docs and the example package included in PEP561

When I build the package locally, the file is included; however, the layout I get in the locally built wheel is slightly different than the published version, so there is definitely some configuration or setuptools/wheel version difference.