chore: List required dependencies

[kesara]

file: requirements.txt is only supported by setuptools>=62.6.

Fixes #1049

[kesara]

The current set of changes uses pyproject.toml more. But this lets us keep requirements in a single place.
The only downside is when installing on the system (without using venv) xml2rfc doesn't install the dependencies under Ubuntu 22.04 LTS.
It works fine when installed within a venv.

Using pip bare is warning users as well:

WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv

[kesara]

It seems like we have to define requirements in setup.cfg as a list to make this particular version setup happy.