set the version number somewhere

[12rambau]

Fix #161

My understanding of the problem is that the version number is not set anywhere.

pyproject.toml uses a dynamic version number but the dynamic estination is not set, xee package has no version variable. the one in ext is using the one from metadata which relies on the pyproject file.

I decided to start this PR by setting where the dynamic version number should come from. Now I need your review to know where the hard coded version number should be set.