Add sources tests/minimext-0.1-*.whl wheels

[stewartmiles]

Is your feature request related to a problem? Please describe.
There are no sources for tests/minimext-0.1-*.whl are in this repository. This is a problem for contributors as there is no way to rebuild / extend these wheels used for testing.

Describe the solution you'd like
It would be great to have the sources in a repo and a way to build these test wheels.

Describe alternatives you've considered
N/A

Additional context
Related to adding support for regular wheels with extensions (see #222)