ui/identity: remove _AsyncValidatedMixin

_AsyncValidatedMixin has been merged into the parent UsernameEditor,
reluctantly. I like the concept of _AsyncValidatedMixin, but what's
happening here is that UsernameEditor has inherited from 3 classes, the
first of which is in urwid, and when the constructor /
super().__init__() status changed in urwid,
_AsyncValidatedMixin.__init__() stopped being called.

OK cool, so maybe we'll just manually run initializers in the case where
it matters.  That's semi-better, but with old urwid we end up calling
_AsyncValidatedMixin.__init__() twice (once directly, once by the urwid
__init__ using super()).

Further workarounds could be employed but at the moment there is one
user of _AsyncValidatedMixin, so just merge it into UsernameEditor.

subiquity/ui/views/identity.py

@@ -53,9 +53,7 @@ class RealnameEditor(StringEditor, WantsToKnowFormField):
             return super().valid_char(ch)
 
 
-class _AsyncValidatedMixin:
-    """Provides Editor widgets with async validation capabilities"""
-
+class UsernameEditor(StringEditor, WantsToKnowFormField):
     def __init__(self):
         self.validation_task = None
         self.initial = None
@@ -63,6 +61,12 @@ class _AsyncValidatedMixin:
         self._validate_async_inner = None
         connect_signal(self, "change", self._reset_validation)
 
+        self.valid_char_pat = r"[-a-z0-9_]"
+        self.error_invalid_char = _(
+            "The only characters permitted in this field are a-z, 0-9, _ and -"
+        )
+        super().__init__()
+
     def set_initial_state(self, initial):
         self.initial = initial
         self.validation_result = initial
@@ -86,15 +90,6 @@ class _AsyncValidatedMixin:
             self.validation_result = await self._validate_async_inner(value)
             self.bff.validate()
 
-
-class UsernameEditor(StringEditor, _AsyncValidatedMixin, WantsToKnowFormField):
-    def __init__(self):
-        self.valid_char_pat = r"[-a-z0-9_]"
-        self.error_invalid_char = _(
-            "The only characters permitted in this field are a-z, 0-9, _ and -"
-        )
-        super().__init__()
-
     def valid_char(self, ch):
         if len(ch) == 1 and not re.match(self.valid_char_pat, ch):
             self.bff.in_error = True