Bump version to 4.0-alpha

This is _not_ yet a feature freeze, and the API should not be considered
stable. The alpha phase will have frequent official testing releases while
we focus on fixing known issues and integrating the remaining features and
API improvements planned for Godot 4.0.

version.py

@@ -3,7 +3,7 @@
 major = 4
 minor = 0
 patch = 0
-status = "dev"
+status = "alpha"
 module_config = ""
 year = 2022
 website = "https://godotengine.org"