bcrypt wheel fix and sklearn from pip

[roflcoopter]

This PR fixes two problems with the wheels:

• Wheel builds are currently broken due to bcrypt not having prebuilt wheels for all architectures, and it requires Rust to build from source.
• Install sklearn from pip instead of apt, since there is a dependency issue between numpy and sklearn from apt.
  Installing sklearn from pip introduces another problem, there are no prebuilt wheels for armv7l (rpi3) and python 3.8.
  Building sklearn from source takes around 10 hours, which makes it impossible to do this via Azure.
  The workaround is the build the rpi3-sklearn Docker container locally, and push to Docker Hub.
  rpi3-sklearn is then used as the base for rpi3-wheels

Closes #473

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