Set required python version to >= 3.8

[mbirtwell]

Fixes #40

This seems to be working for me, so I thought I'd share it. Without this I can't install juliacall using poetry for python versions other than 3.8.

[cjdoris]

Thanks. Contrary to what #40 says, ~=3.8 is equivalent to >=3.8,<4, so it should be installable with 3.9 or 3.10 or ... . However some package managers (definitely uv the last time I checked) don't handle some compat bounds on python correctly. I've hit against this so many times!

[mbirtwell]

~=3.8 is equivalent to >=3.8,<4, so it should be installable with 3.9 or 3.10 or ...

Indeed. Sorry I just assumed that poetry was doing the right thing.