FIX: account for changes in py310

In python/cpython#28420 the order of setting the
default values on the Namespace object and processing the Actions was reversed.
This means that in the `__call__` method of `_FlagAction` the `_flags`
attribute has not yet been put on the namespace.

This change makes `_FlagAction.__call__` forgiving if the `_flags` attribute
does not exist (by creating it!).

Author: tacaswell

traitlets/config/loader.py

@@ -928,6 +928,8 @@ def __init__(self, *args, **kwargs):
 
     def __call__(self, parser, namespace, values, option_string=None):
         if self.nargs == 0 or values is Undefined:
+            if not hasattr(namespace, '_flags'):
+                namespace._flags = []
             namespace._flags.append(self.flag)
         else:
             setattr(namespace, self.alias, values)