Drop Python 3.7 and 3.8 for consistency with mdformat

[KyleKing]

CI is currently failing because mdformat-myst still attempts to test against Python 3.7, but that version is no longer supported by latest GitHub Actions nor mdformat (hukkin/mdformat@63bd6b6)

I'll open a PR to carry over the mdformat changes here and unblock merging #40 this week unless someone else has more availability