* vdirsyncer: standardize derivation
Use toPythonApplication and use callPackage from python-packages.nix.
Make vdirsyncerStable somewhat functional again, but mark it as broken
for Python 3.6 or higher.
* vdirsyncer: use buildPythonPackage as it's a package now
* vdirsyncer: move to python-modules/
* vdirsyncer: Move disabled logic into expression
