package/python-packaging: depend on python3

The latest version of this package requires python 3.6+.
The only reverse dependency, pytest, already requires python3.

Fixes:
 - http://autobuild.buildroot.net/results/217157f4dd9fd56d0a13d47e6373f4c478f26757/

Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
[yann.morin.1998@free.fr: add note about pytest]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/python-packaging/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_PYTHON_PACKAGING
 	bool "python-packaging"
+	depends on BR2_PACKAGE_PYTHON3
 	select BR2_PACKAGE_PYTHON_PYPARSING # runtime
 	help
 	  Core utilities for Python packages.