Manage the version number with setuptools_scm

[marcelm]

With this, the version number is obtained from the most recent Git tag. It works in the following way.

• When the current HEAD has tag v3.141, blr --version prints 3.141.
• If the most recent tag is v3.141, but 5 extra commits have been made since then, the output is v3.142.dev5+ged3560f (the minor version is incremented, the dev5 part describes the number of commits since the most recent tag, and the string after +g is the HEAD commit hash.)
• If local, uncommitted changes exist, the current date is appended to the version number, as in .d20200616.

I also highly recommend to tag a version number soon.

[pontushojer]

Awesome! This will be very useful, I say we tag directly following merging this PR.