There is no need to be remotely fetching code for a simple 100 lines
file of code which should never change anymore.

We use the first method from the documentation at
https://packaging.python.org/en/latest/guides/single-sourcing-package-version.

Thanks to Weiliang Li (Github: https://github.com/kigawas) for pointing
out the source build was broken and proposing a (different) fix.

Install python-bip32 from source to avoid
#41 from happening again.

Unfortunately building coincurve on MacOS keeps failing for esoteric
reasons and i don't have infinite time, so on MacOS first install
coincurve wheels before installing python-bip32.