This removes radicale-py2, the Py2 variant, and renames radicale-py3 to
radicale.

This also makes a number of changes:
* Actually use the Python package build system (from python3-package.mk)
* Download source from PyPI instead of GitHub git repo
* Remove unnecessary PKG_DEFAULT_DEPENDS definition
* Depend on python3-urllib instead of python3-email (now that urllib is
  separate from python3-light and has python3-email as a direct
  dependency)
* Move package description from menuconfig help to the actual
  description field
* Remove unnecessary preinst script (default prerm will stop the
  service now that the package name matches the init.d script name)
* Remove unnecessary lib/upgrade/keep.d entry (changed conffiles are
  preserved by sysupgrade by default)
* Remove unnecessary postinst script (Python build system will set the
  correct shebang)

Signed-off-by: Jeffery To <jeffery.to@gmail.com>