Find out which bundles are available automatically

[dhalbert]

Right now circup needs to be updated manually whenever we change which bundles are being built. Two possibilities:

• Make it easier to update circup. Make it depend on the contents of https://github.com/adafruit/circuitpython-build-tools/blob/main/circuitpython_build_tools/target_versions.py.
  OR
• Make it unnecessary to update circup. Have it fetch which bundles are available when it runs. Could be from the file above, or from a data file placed in S3 (maybe just a copy of the above). https://adafruit-circuit-python.s3.amazonaws.com/bundles/...