Deprecate NEED_PYTHON variable.

Deprecate NEED_PYTHON variable.
Require python_pkg_setup() in EAPI >=4.
Ensure that sanity checks in python_pkg_setup() and python_set_active_version() are performed for binary packages.
Improve handling of arguments in some functions.
Support flags specific to given Python ABIs.
Improve sanity checks in some functions.
Improve error messages in some functions.
Support --base-path option in python_get_includedir(), python_get_libdir(), python_get_sitedir() and python_get_library().
Allow multiple paths in argument of --PYTHONPATH option of python_execute_nosetests(), python_execute_py.test() and python_execute_trial().
Simplify python_mod_optimize() and python_mod_cleanup() in EAPI >=4.
Clean unneeded code in python_version(), python_mod_exists() and python_tkinter_exists().