Update for pyproject.toml-style

[miketheman]

With the uptick in pyproject.toml usage, update library without setup.py implementation.

[kjaymiller]

Just came in after a long conversation with some other maintainers hoping to do some automated calver style without a setup.py

[di]

Unfortunately this library is basically just a setuptools plugin, so it doesn't have much use in a world without setup.py, mainly because PEP 643 says Version cannot be dynamic.

I think my preference would be to make https://pypi.org/project/bump/ take a --calver flag that just sets the version it to the current date, which could be used prior to building a distribution.