There's a merged pull request on the void-packages repository

[RFC] create build_helper for python3 cross-building
https://github.com/void-linux/void-packages/pull/38911

Description:
- common/build-helper/python3.sh: add python3 helper
- common/{,environment}/build_style/python3.sh: use python3 build_helper
- brltty: use python3 build_helper, clean up
- i2c-tools: use python3 build_helper
- libpwquality: use python3 build_helper
- tlsh: use python3 build_helper

My hope with this is to help with cross-building non-python packages that have python3 bindings/modules, but it can be used similar to the `rust` `build_helper` for various common constants, if needed.

One thing I'm not sure on is the usage with the `python3-module` `build_style`. It would cut down on the need to keep those env var definitions up-to-date in multiple places, but I'm open to opinions about it.

There's probably more packages that are marked `nocross`/`broken` because of python3 cross stuff that I didn't find.

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#### Testing the changes
- I tested the changes in this PR: **briefly**
    - packages changed seem to build the same thing as before, but weren't extensively tested