package/python-cryptography: depend comment on BR2_PACKAGE_PYTHON3

The user shouldn't see the comment on the python2 menu.

Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/python-cryptography/Config.in

@@ -17,4 +17,5 @@ config BR2_PACKAGE_PYTHON_CRYPTOGRAPHY
 	  https://cryptography.io
 
 comment "python-cryptography needs a toolchain w/ C++"
+	depends on BR2_PACKAGE_PYTHON3
 	depends on !BR2_INSTALL_LIBSTDCPP