bump required python minor version

[sbryngelson]

I made it require Python 3.9. I know 3.6 doesn't work. I know 3.9 does. I do not know about 3.7 or 3.8. For the math.prod this has a documented update somewhere, but 3.6 also doesn't work once this is replaced with a 3.6-compatible command (something wrong with the subprocesses call).

Closes #265

[henryleberre]

lgtm 🚀

~/MFC $ vermin --eval-annotations --backport argparse --backport dataclasses --backport typing --no-parse-comments toolchain/
Minimum required versions: 3.9
Incompatible versions:     2