PEP 440 conformance for development builds

Recent setuptools releases refuse to build packages with invalid version
strings. So instead of using the bare commit hash as the version, we now
build proper version strings like X.Y.Z.dev-git.COMMIT for CI development
builds and X.Y.Z for release builds (same as before).

.gitlab-ci.yml

@@ -14,7 +14,7 @@ before_script:
   - uname -a
   - python3 -m pylint --version
   - python3 -m coverage --version
-  - echo "${CI_COMMIT_TAG}" | grep -qE "v[0-9]+[.][0-9]+[.][0-9]+.*" && export PACKAGE_VERSION="${CI_COMMIT_TAG#v}" || export PACKAGE_VERSION="${CI_COMMIT_SHA}"
+  - export PACKAGE_VERSION="$(git describe | sed -E -n -e 's/^v([0-9.]*)$/\1/p' -e 's/^v([0-9.]*)-([0-9]*)-g([0-9a-z]*)$/\1.dev+git.\3/p' | grep .)"
 
 .build:
   stage: build