Add workaround for debugpy install issue to contrib docs. (#1835)

When following the Quick Start instructions in the contributing docs, I
was getting a "WheelFileValidationError" on installation of debugpy
which was blocking the installation of a number of other deps. Google
turned up this [GitHub
issue](https://github.com/microsoft/debugpy/issues/1246) indicating a
regression in Poetry 1.4.1 and workarounds.

This PR updates the contrib docs noting the issue and the workarounds.