Allow parse_float to return values with append attr

Author: hukkin

README.md

@@ -95,7 +95,7 @@ assert toml_dict["precision-matters"] == Decimal("0.982492")
 Note that `decimal.Decimal` can be replaced with another callable that converts a TOML float from string to a Python type.
 The `decimal.Decimal` is, however, a practical choice for use cases where float inaccuracies can not be tolerated.
 
-Illegal types include `dict`, `list`, and anything that has the `append` attribute.
+Illegal types are `dict` and `list`, and their subtypes.
 Parsing floats into an illegal type results in undefined behavior.
 
 ## FAQ<a name="faq"></a>

src/tomli/_parser.py

@@ -216,10 +216,9 @@ def append_nest_to_list(self, key: Key) -> None:
         last_key = key[-1]
         if last_key in cont:
             list_ = cont[last_key]
-            try:
-                list_.append({})
-            except AttributeError:
+            if not isinstance(list_, list):
                 raise KeyError("An object other than list found behind this key")
+            list_.append({})
         else:
             cont[last_key] = [{}]