Improve handling of get_version

[blink1073]

Problem

We are currently using either the deprecated python setup.py --version or the slow build-a-wheel-and-extract-metadata method to get the current version.

Proposed Solution

Use the approach recommended by setuptools_scm to get the version:

from importlib.metadata import version, PackageNotFoundError

try:
    __version__ = version("package-name")
except PackageNotFoundError:
    # package is not installed
    pass

We'd have to install the package in editable mode so that the version can remain dynamic.

[blink1073]

Let's go ahead and bump our minimum Python version to 3.8 for this as well to avoid the backport, since that is the version currently supported by https://numpy.org/neps/nep-0029-deprecation_policy.html

[blink1073]

Since we're no longer installing the package, let's not do this. cf #449