package/python-pycups: needs python3

As stated in README:

Python 3 or later is required since Python 2 is unsupported since
2020-01-01.

Fixes:
 - http://autobuild.buildroot.org/results/3c8db83a2d1ae094fc718947a508e35e940121ce

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/python-pycups/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_PYTHON_PYCUPS
 	bool "python-pycups"
 	depends on BR2_PACKAGE_CUPS
+	depends on BR2_PACKAGE_PYTHON3
 	help
 	  Python bindings for libcups.