Bump version to 3.0-alpha

WARNING: This branch is unstable and should not be used in
production for the time being.
See https://godotengine.org/article/warning-head-going-unstable
for more details.

To get the previous 2.2-alpha unreleased version, you can checkout
the '2.2' branch.

version.py

@@ -1,5 +1,5 @@
 short_name = "godot"
 name = "Godot Engine"
-major = 2
-minor = 2
+major = 3
+minor = 0
 status = "alpha"