Merge branch 'master' into core-updates

Merge branch 'python'

guix: python: Create module installation path and add it to PYTHONPATH during
  the installation phase.

* guix/build/python-build-system.scm (get-python-version): New procedure.
* guix/build/python-build-system.scm (install): Create and add path.
* gnu/packages/python.scm (python-setuptools): Drop path creation code.

guix: python: Add build phase and factor out calls to setup.py.

* guix/build/python-build-system.scm (call-setuppy): New procedure.
* guix/build/python-build-system.scm (build): New procedure.
* guix/build/python-build-system.scm (check, install): Use call-setuppy.
* guix/build/python-build-system.scm (%standard-phases): Add call to build.

gnu: python: Honour #:tests? and #:test-target in build system.

* guix/build/python-build-system.scm (check): Use named parameters tests?
    and test-target (default now: "test" instead of "check").

guix: python: Switch to python-wrapper as the default version for the python
  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.

python-build-system: add a check phase.

* guix/build/python-build-system.scm (check): New procedure.
  (%standard-phases): Use it.

Add 'python-build-system'.

* guix/build-system/python.scm, guix/build/python-build-system.scm: New files.
* Makefile.am (MODULES): Add them.