RHEL8/EPEL brings `importlib-metadata` 0.23 as a package. Since this is
a backport of a Python stdlib package (starting with Python 3.9), we can
assume its API to be quite stable (furthermore, `gptsum` only uses very
basic functionality ofo the library), so let's depend on 0.23 (or
higher) without any other bounds (since 'or' version bounds can't be
defined in Python, it seems).

See: python-poetry/poetry#3840

This is a first step to add RPM building to the project. It adds a new
Nox session which doesn't install dependencies as pinned by Poetry (in
`poetry.lock`), but instead hard-codes some versions/constraints as
found in CentOS 8 and EPEL.

Since the original tests code was not compatible with these older
versions of packages (e.g., `pytest`), some code changes were required,
either based on import names being (un)available, or based on version
checks.