[CT-1290] Check root cause of not supporting Python 3.7.0/1

[nathaniel-may]

dbt-core does not run with Python 3.7.0, Python 3.7.1 (issue for removing support / (merged PR)).

Looking at this comment, it's possible we could undo this restriction on any dbt versions that support Mashumaro 3.0+ which as of now includes 1.3.latest.

The task is to undo the restriction on main and see if dbt runs. If it does, we can remove the Python version restriction, and backport to 1.3.latest.

Because there are not many people using these old versions of Python this is a low impact change.

[peterallenwebb]

We no longer support 3.7