Find out which bundles are available automatically

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/...`

Start by comparing circuitpython_build_tools/target_versions.py with the bundle data under the stated S3 path. Determine whether circup should depend on that file or fetch available bundles at runtime; the issue is done when bundle changes no longer require a manual circup update.

Written by the indexing model from the issue text.