package/python-asgiref: drop Python 2 support

Asgiref doesn't support Python 2 since version 2.0.0.
All the recursive reverse dependencies already support Python 3 only.

Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/python-asgiref/Config.in

@@ -1,4 +1,5 @@
 config BR2_PACKAGE_PYTHON_ASGIREF
+	depends on BR2_PACKAGE_PYTHON3
 	bool "python-asgiref"
 	help
 	  ASGI is a standard for Python asynchronous web apps and