Bump version to 3.1.2-stable

Tons of bugfixes have been cherry-picked since 3.1.1 in April,
this should make the 3.1 branch a lot more reliable to ship games.

Thanks to all contributors! <3

version.py

@@ -3,7 +3,7 @@ name = "Godot Engine"
 major = 3
 minor = 1
 patch = 2
-status = "rc"
+status = "stable"
 module_config = ""
 year = 2019
 website = "https://godotengine.org"