build-system/python: Export 'python-version'.

* guix/build/python-build-system.scm (get-python-version): Rename to...
(python-version): ... this.  Update callers.  Make public.
* gnu/packages/gstreamer.scm (python-gst)[arguments]: Adjust accordingly.
* gnu/packages/machine-learning.scm (ghmm)[arguments]: Likewise.
* gnu/packages/openldap.scm (389-ds-base)[arguments]: Import (guix
build python-build-system).  Use 'python-version'.
* gnu/packages/package-management.scm (conda)[arguments]: Use
'python-version'.