ci: run mypy in Python 3.8 mode instead of Python 3.7 mode

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Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (b7ed503) compared to base (6281501). Patch has no changes to coverable lines.

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[agoose77]

As discussed on Slack, the lower-bound on this NumPy version exceeds the last version that supported Python 3.7. We probably need to change this to an upper bound (with LB if known) of <1.22.0, or split the CI into two jobs if we need to test newer stubs for 3.10.

[alexander-held]

The numpy version required for mypy to run without additional # type: ignore additions is numpy>=1.22. Since numpy==1.22 is Python 3.8+, the issue with positional-only parameters (which are also Python 3.8+) is not avoidable in Python 3.7 runs of mypy which have been used so far.

As cabinetry is likely to drop support for Python 3.7 soon (pyhf has already done this in https://github.com/scikit-hep/pyhf/pull/2044), we can drop the Python 3.7 runs of mypy in favor of running with 3.8, which will fix the CI issue.

* run mypy with Python 3.8 instead of 3.7 in pre-commit
* set default Python version for mypy to 3.8 
* updated pre-commit