Update pyproject.toml

[rhiannonlynne]

Fix pyproject.toml to let pypi distribution work properly. The "License" line in pyproject.toml was causing it to be rejected by pypi.

[rhiannonlynne]

I was able to tag and test this on a fork, so figured it could come back to rubin_sim directly from my fork.
After merging this PR and tagging (1.3.2) rubin_sim, the pypi upload will hopefully be triggered automatically (as well as a new, independent, conda build). Might have to go check in on the conda build after that.
One note: pypi has pyoorb, built with pip and no open orb-data option -- but conda has openorb, built with conda-forge.

[rhiannonlynne]

Can test the testPyPI installation with
pip install -i https://test.pypi.org/simple/ "rubin-sim[test]" --extra-index-url https://pypi.python.org/simple
(although do need to also install pyoorb data requirements, which is easiest by mamba install openorb-data and mamba install openorb-data-de405)