dependency issue that involves python 3.7, openbabel 2.4 and numpy 1.21

[beef-broccoli]

vulnerability issue with numpy 1.21, 1.22 recommended, which requires python >=3.8

however, openbabel 2.4 seems to be more stable than openbabel 3.x. openbabel 2.4 appears to work only with python <= 3.7

Set python to >= 3.7 for now. In the future, might update to python >=3.8, which requires the use of openbabel 3.x, which needs more testing on stability.

[beef-broccoli]

#20 v1.3.0 support Python 3.8, although not extensively tested