Make development version strings PEP 440 compliant

With current pip (23.1.2) and setuptools (67.7.2) versions, installation
from Git was failing with "Invalid version: '7.0.0-17-ge57cf1e7'"
message. The issue can be reproduced by running the following command
with the latest pip and setuptools installed:

  $ pip install git+https://code.ungleich.ch/ungleich-public/cdist.git@e57cf1e70a3818180b3f54e1e0364ff3b6bbd9cc#egg=cdist

bin/cdist-build-helper

@@ -534,7 +534,8 @@ eof
     ;;
 
     version)
-        printf "VERSION = \"%s\"\n" "$(git describe)" > cdist/version.py
+        target_version="$(git describe | sed 's/-/.dev/' | sed 's/-/+/g')"
+        printf "VERSION = \"%s\"\n" "${target_version}" > cdist/version.py
     ;;
 
     target-version)