Modernize python-distutils.mk installdir handling (since supported even in oldstable now): Use --install-layout=deb (and drop --prefix=/usr) by default, and use --install-lib option (not --install-purelib/--install-platlib, and only when DEB_PYTHON_DISTUTILS_INSTALLDIR_SKEL is set).  Deprecate cdbs_expand_python_distutils_installdir, and have it provide .../dist-packages dir (not .../site-packages) by default to try cope with packages relying on it for non-skel use. Closes: bug#565973. Thanks to Matthias Klose and Piotr Ożarowski.
Fix error message to talk about multiple python implementations (not packaging systems).
Tidy python-vars.mk: Add/improve comments.
Tidy python-vars.mk: Reorder variables to list legacy before modern, and cdbs_curpkg-resolved last.
Deprecate DEB_CONFIGURE_SCRIPT_ENV, to allow better override control over related variables through its successor, cdbs_autotools_configure_env.
Rename cdbs_python_pysupport → cdbs_pythonsystem_pysupport, and deprecate the former.
Rename cdbs_python_pycentral → cdbs_pythonsystem_pycentral, and deprecate the former.
Rename cdbs_python2 → cdbs_pythonsystem_python2, and deprecate the former.
Rename cdbs_python3 → cdbs_pythonsystem_python3, and deprecate the former.
Refactor python snippets: Expand implementations from list.
Refactor python snippets: Handle flavors (not versions).
Rename cdbs_python2_builddeps → cdbs_python_builddeps_python2, and deprecate the former.
Rename cdbs_python3_builddeps → cdbs_python_builddeps_python3, and deprecate the former.
Refactor python snippets: Check actual need for build-dependencies in cdbs_python_builddeps (not before).
Refactor python snippets: Expand packaging systems from list.
Refactor python snippets: Handle runtimes separate from flavors.
Support building python modules for pypy

CDBS will trigger build of python module for pypy, if it finds pypy-
prefix in binary package name. This patch tries to be compatible with
existing tasks done by CDBS for python and python3 variants where ever
it is possible.
Enable pypy tests.
Update autotools (with autoreconf -f -i).
Add FIXMEs and TODOs.