python-crossbar: drop Python 2 support

Upstream is now Python 3 only.

Quoting the maintainer [1]: "the last version of crossbar with python 2
support: pip install crossbar==18.4.1".

[1] https://github.com/crossbario/crossbar/issues/1332

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Asaf Kahlon <asafka7@gmail.com>
Cc: Mauro Condarelli <mc5686@mclink.it>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/python-crossbar/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_PYTHON_CROSSBAR
 	bool "python-crossbar"
 	depends on BR2_INSTALL_LIBSTDCPP # python-cryptography
+	depends on BR2_PACKAGE_PYTHON3
 	# All the following dependencies are runtime dependencies
 	select BR2_PACKAGE_PYTHON_ATTRS
 	select BR2_PACKAGE_PYTHON_AUTOBAHN
@@ -48,3 +49,4 @@ config BR2_PACKAGE_PYTHON_CROSSBAR
 
 comment "python-crossbar needs a toolchain w/ C++"
 	depends on !BR2_INSTALL_LIBSTDCPP
+	depends on BR2_PACKAGE_PYTHON3