This version change requires a migration to a new API. See
https://pyo3.rs/v0.21.2/migration#from-020-to-021
This will fix the annoying warnings added when using the recent rust
nightly:
> warning: non-local `impl` definition, they should be avoided as they
go against expectation
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
