make Kwant versioning work with `git archive` output
A valid way of installing the latest master
vesion of Kwant is from a ZIP archive from Gitlab or Github, which are created using git archive
The current versioning logic does not work in these case.
We can make the versioning work in this case by adding a string to _kwant_version
that is expanded when running git archive
, by adding the following line to .gitattributes
: kwant/_kwant_version.py export-subst