Strip trailing _ from config members, to bypass Python reserved words

Example of adding an `async` boolean option that can be read through `config['async']` or `config.async_`:

    class ExampleConfig(Config):
        async_ = Type(bool, default=False)

mkdocs/config/base.py

@@ -69,6 +69,8 @@ def post_validation(self, config: Config, key_name: str) -> None:
         """
 
     def __set_name__(self, owner, name):
+        if name.endswith('_') and not name.startswith('_'):
+            name = name[:-1]
         self._name = name
 
     @overload
@@ -123,7 +125,7 @@ def __init_subclass__(cls):
         schema = dict(getattr(cls, '_schema', ()))
         for attr_name, attr in cls.__dict__.items():
             if isinstance(attr, BaseConfigOption):
-                schema[attr_name] = attr
+                schema[getattr(attr, '_name', attr_name)] = attr
         cls._schema = tuple(schema.items())
 
         for attr_name, attr in cls._schema: