Ship intermediate channel_v4 for python 3.8 libraries

Not ideal, but the only way to support migration to python 3.8 for all packages
is to provide an intermediate channel_v4 with required metadata to support
already existing dependencies/libraries on python 3.8 plugin_host.

Note: They are maintained at https://github.com/packagecontrol/channel

Package Control.sublime-settings

@@ -3,6 +3,11 @@
 	// The repositories from these channels are placed in order after the
 	// repositories from the "repositories" setting
 	"channels": [
+		// channel_v4 for python 3.8 compatible libraries to enable plugins
+		// to migrate to python 3.8 until packagecontrol.io supports new scheme.
+		// Note: Must always be located before default channel in the list!
+		"https://packagecontrol.github.io/channel/channel_v4.json",
+		// default channel for packages
 		"https://packagecontrol.io/channel_v3.json"
 	],