vendored pgpy breaking with six==1.11.0

latest tests are breaking because of a new six release: https://0xacab.org/leap/leap_mx/-/jobs/20310

It's the same issue that made drebs pin the six version used for tests in soledad: https://github.com/SecurityInnovation/PGPy/issues/217

I would say this is an issue in six, but I haven't dedicated too much time. For me the obvious solution for now is to vendor also the working form from six==1.10.0