Package PyO3 0.20.2

Hello,

I'm trying to get pythonize updated to 0.20.0 for a Matrix Synapse update. I'm not sure what fraction of the dependent packages need to be evaluated to do this update.

• testing_logger and serde were previously in this MR, but they've already been packaged, so I've dropped them
• pyo3 looks like it's moved to dual Apache/MIT license. (question: should any LICENSE be listed in the debian/copyright? I would think we don't need to worry about that file separately)
• pyo3-file has a simple patch that gets it to work with pyo3 0.20.0
• pep440_rs and pep508_rs are in a weird state, despite being written by the same author. I think this untangles that situation, and I've checked that pyproject-toml, the sole reverse dependency I could find, already seems to fails its autopkgtests, but fails no additional ones with this change.