Use upstream version of bitarray if/when they publish wheels

[hardbyte]

We are using a fork of bitarray which publishes binary wheels to ensure wider support. This was introduced in #248

The upstream PR (to introduce building the wheels at least) is https://github.com/ilanschnell/bitarray/pull/76

This issue is for us to move back to the upstream version of bitarray once binary wheels are available in pypi for MacOS, Windows and Linux platforms.