gnu: Add python-dateutil, python2-dateutil.

* gnu/packages/python.scm (python-dateutil, python2-dateutil): New variables.

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.

gnu: Add python-setuptools, python2-setuptools.

* gnu/packages/python.scm (python-setuptools, python2-setuptools):
    New variables.

guix: python: Do not import %standard-phases from gnu-build-system.

* guix/build-system/python.scm (python-build): Drop module gnu-build-system.
    Thanks to Ludovic Courtès <ludo@gnu.org>.

guix: python: Add parameter #:phases to build system.

* guix/build-system/python.scm (python-build): Use parameter #:phases.

guix: python: Add package-with-python2, a procedure rewriting a package
  to compile with Python 2 instead of the default Python 3.

* guix/build-system/python.scm (default-python2, package-with-explicit-python,
    package-with-python2): New procedures.
* guix/build-system/python.scm (python2-pytz, python2-babel): Use
    package-with-python2.

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: Disable tests in python2-pytz and python2-babel.

* gnu/packages/python.scm (python2-pytz, python2-babel): Disable tests.

gnu: python: Disable tests in pytz and babel.

* gnu/packages/python.scm (python-pytz, python-babel): Disable tests.

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").

gnu: git: Switch back to Python 2.

* gnu/packages/version-control.scm (git): Switch back to Python 2.

gnu: python: Implement the python naming scheme for pytz and babel.

* gnu/packages/python.scm (pytz): Rename this ...
* gnu/packages/python.scm (python-pytz): ... to this
* gnu/packages/python.scm (python2-pytz): New variable, input python-2.
* gnu/packages/python.scm (babel): Rename this ...
* gnu/packages/python.scm (python-babel): ... to this
* gnu/packages/python.scm (python2-babel): New variable, input python-2.

gnu: subversion: Switch back to Python 2.

* gnu/packages/version-control.scm (subversion): Switch back to Python 2.

gnu: qemu: Switch back to Python 2.

* gnu/packages/qemu.scm (qemu): Switch back to Python 2.

gnu: texlive: Switch back to Python 2.

* gnu/packages/texlive.scm (texlive): Switch back to Python 2.

gnu: rubber: Switch back to Python 2.

* gnu/packages/texlive.scm (rubber): Switch back to Python 2.

gnu: mesa: Switch back to Python 2.

* gnu/packages/xorg.scm (mesa): Switch back to Python 2.

gnu: libxml2: Switch back to Python 2.

* gnu/packages/xml.scm (libxml2): Switch back to Python 2.

gnu: zziplib: Switch back to Python 2.

* gnu/packages/zip.scm (zziplib): Switch back to Python 2.

gnu: bazaar: Switch back to Python 2.

* gnu/packages/version-control.scm (bazaar): Switch back to Python 2.