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
There are a couple of things we need to be careful of here:
1. The current python code does no validation when loading from the DB,
so we need to be careful to ignore such errors (at least on jki.re there
are some old events with internal metadata fields of the wrong type).
2. We want to be memory efficient, as we often have many hundreds of
thousands of events in the cache at a time.
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Co-authored-by: Quentin Gliech <quenting@element.io>