Add support for Pyproject toml circup dependencies

[FoamyGuy]

This change makes circup look for a circup section with circup_dependencies inside of the pyproject.toml file and use it as an additional list of dependencies that is separate from requirements.txt. This is to be used for circuitpython libraries that are included in bundle, but not deployed to pypi and thus aren't valid in requirements.txt

Here is an example declaration of one such dependency this was tested with:

[circup]
circup_dependencies = ["cedargrove_midi_tools"]

[FoamyGuy]

this offers a solution for: #181

[tannewt]

Neat! Thank you!