Signed-off-by: Gabor Boros <[email protected]>

We previously made use of the pkg_resources module to determine the
package version (as defined by setuptools-scm) in __about__.py. This
module is included in setuptools, which as of Python 3.12 has been
removed from the standard library and is now its own third-party
package.

There are multiple ways to address this, one being to include
setuptools in the install_requires list for Python 3.12 and
later. Another, also listed in the Python Packaging User Guide, is to
instead use importlib.metadata.version. Since this approach works for
all Python versions we intend to support (that is, from 3.8 forward),
this is the better solution.

Thus, refactor __about__.py to use importlib, and add Python 3.12 to
the test matrix.

Note that Tutor itself does not support Python 3.12 prior to version
17, so we cannot support the combination of Python 3.12 and Tutor
version 16 or earlier.

Reference:
https://packaging.python.org/en/latest/guides/single-sourcing-package-version/