Pin project Python version to minor version number

Previously the configuration in the pyproject.toml file only pinned the major version number of Python. Although in
theory a strict adherence to SemVer would ensure that this was no problem, in practice changes that are breaking for our
usage have been introduced in minor version bumps of Python. So for the stability of the project infrastructure we will
now pin to the minor version. The hope is that leaving the patch version number unpinned will allow some flexibility to
contributors in regards to which version they must have installed locally while not resulting in instability.

Author: per1234