fix: reference docs

[maddyobrienjones]

works fine when generated locally, but on readthedocs, the reference page doesn't show any of the module information

https://pyisic.readthedocs.io/en/latest/topics/reference.html

https://readthedocs.org/projects/pyisic/builds/15023633/
problem seems to be that docs are generated with python 3.6 when they should be generated with python 3.7 (3.6 does not have the dataclasses module)