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* Use minijinja's pycompat mode for python methods

* fix: cargo fmt lint for pre commit

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Co-authored-by: Armin Ronacher <armin.ronacher@active-4.com>

Close #2007

- Refactor code to allow supporting multiple versions of the
generate.proto at the same time
- Add v3/generate.proto (ISO to generate.proto for now but allow for
future changes without impacting v2 backends)
- Add Schedule trait to abstract queuing and batching mechanisms that
will be different in the future
- Add SchedulerV2/V3 impl