package/python3: add optional support for lib2to3

package/python3: add optional support for lib2to3

Kodi is in transition to support python3 instead of python2:
https://kodi.wiki/view/General_information_about_migration_to_Python_3

"For Kodi 18 (Leia), only addons that are compatible with both Python 2
 and 3 will be accepted to the official addon repository."

Some of these addons depend on the Kodi addon script.module.future to
provide support for both python versions.

The script.module.future addon contains python-future:
https://kodi.wiki/view/General_information_about_migration_to_Python_3#Future
which in turn needs lib2to3 to be included in the target build of
python3: http://python-future.org/automatic_conversion.html

Kodi addons depending on the script.module.future addon are crashing on
buildroot due to lib2to3 missing in the build.

LibreELEC added lib2to3 to python3 to fix the problem:
https://forum.libreelec.tv/thread/21239-lib2to3-pgen2-parse-missing/
https://github.com/LibreELEC/LibreELEC.tv/pull/4146

This patch provides the Config.in option to be used by Kodi 19.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[yann.morin.1998@free.fr:
  - fix conflicts due to local changes in author's tree
  - fix typ in variabl name (PYTHON_CONF_OPTS -> PYTHON3_CONF_OPTS)
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>