change versioning approach

[ilaflott]

It's hard-coded in like, three different places:

https://github.com/ilaflott/epmt/blob/47a9c042bbd76d3cc4ebb11224731048cd448a7b/src/setup.py#L4

https://github.com/ilaflott/epmt/blob/47a9c042bbd76d3cc4ebb11224731048cd448a7b/src/pyproject.toml#L12

https://github.com/ilaflott/epmt/blob/47a9c042bbd76d3cc4ebb11224731048cd448a7b/src/epmt/epmtlib.py#L26

This can be tweaked to be one spot with typical versioning approaches