Bump version to 3.1-alpha

IMPORTANT: This means that the master branch is now considered feature-complete
for the upcoming 3.1 release, and thus in *feature freeze*.

Unless explicitly allowed by project maintainers, no new feature PRs will be
considered for merge until Godot 3.1-stable is released. Current PRs made
before the feature freeze will still be reviewed and potentially merged before
the beta stage, if deemed satisfactory.

version.py

@@ -2,5 +2,5 @@ short_name = "godot"
 name = "Godot Engine"
 major = 3
 minor = 1
-status = "dev"
+status = "alpha"
 module_config = ""