Fix python-distutils.mk and python-sugar.mk to use unversioned runtime for default flavor (see bug#377964).
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.
Introduce new variables DEB_CONFIGURE_SCRIPT_ENV_BUILDFLAGS cdbs_autotools_configure_env, and deprecate DEB_CONFIGURE_SCRIPT_ENV: supports early and more granular override.
Rename cdbs_python_pysupport → cdbs_pythonsystem_pysupport (and simply drop the former: unused according to codesearch.debian.net).
Rename cdbs_python_pycentral → cdbs_pythonsystem_pycentral (and simply drop the former: unused according to codesearch.debian.net).
Rename cdbs_python2 → cdbs_pythonsystem_python2 (and simply drop the former: unused according to codesearch.debian.net).
Rename cdbs_python3 → cdbs_pythonsystem_python3 (and simply drop the former: unused according to codesearch.debian.net).
Refactor python snippets: Expand implementations from list.
Refactor python snippets: Rename per-flavor variables python_*_packages → _python_*_packages - i.e. internalize them.
Drop deprecated (and unused according to codesearch.debian.net) build target python-install-py python-install-% python-clean-%, and mention bug#759709 for python-build-stamp-%.
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.