python3: make it exclusive from python

In Buildroot, we do not support installing both Python 2.x and Python
3.x on the target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/python3/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_PYTHON3
 	bool "python3"
+	depends on !BR2_PACKAGE_PYTHON
 	depends on BR2_USE_WCHAR
 	# uses fork()
 	depends on BR2_USE_MMU