Add wagtail 6.3 back into tox testing

Author: nickmoreton

tox.ini

@@ -1,8 +1,8 @@
 [tox]
 envlist =
   # Django versions with their respectively supported Python versions and the most recent Wagtail LTS
-  py{39,310,311,312}-django42-wagtail70
-  py{310,311,312,313}-django{51,52}-wagtail70
+  py{39,310,311,312}-django42-wagtail{63,70}
+  py{310,311,312,313}-django{51,52}-wagtail{63,70}
   # Old Wagtail versions with the oldest Django LTS and Python
   py39-django42-wagtail52
 isolated_build = True