[MRG+1] Remove support for Python 3.7

[aaronreidsmith]

Description

Python 3.7 is due to be end-of-lifed on June 27, 2023. This PR preps us to remove support for it.

Think this will depend on alkaline-ml/pmdarima-docker#26

Type of change

• Python version removal

How Has This Been Tested?

3.7 tests no longer exist, everything else still works

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes