Dsps

Require python>=3.9 and change API of diffburst

Rescale mass-metallicity-redshift in [mzr.py](https://github.com/ArgonneCPAC/dsps/blob/b7134d9bf02db3db5b78b5fadc53c178fe3cd2bc/dsps/metallicity/mzr.py) to be based on absolute metallicity rather than Z/Zsun, updating the convention according to the one adopted in the new SSP SEDs https://github.com/ArgonneCPAC/dsps/pull/75

Rename dsps.metallizity.mzr.DEFAULT_MZR_PDICT to dsps.metallizity.mzr.DEFAULT_MET_PDICT. See https://github.com/ArgonneCPAC/dsps/pull/71.

Bug-fix release improves numerical stability of calls to the sigmoid functions, resolving some issues leading to NaNs in gradients

Bug fix release addresses edge case wherein FSPS is accidentally assumed to be installed and configured