build system (switches to Python 3) and compute python-version instead of
passing it as a parameter.
* guix/build-system/python.scm (default-python): Switch to python-wrapper.
* guix/build-system/python.scm (python-build): Drop parameter #:python-version.
* guix/build/python-build-system.scm (wrap): Compute python version from input.
This fixes a circular dependency introduced in commit acc26ff ("gnu: Add
pytz.").
* guix/build-system/python.scm (default-python): New procedure.
(python-build): Use it.