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:/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"
  ],