ENH: avoid subclassing builtin dict

[neutrinoceros]

PR Summary

problem

Subclassing builtin types like dict is error prone because overriden methods are mostly ignored.
This can lead to very subtle bugs where custom classes don't behave as one would expect

class DoubleDict(dict):
    def __setitem__(self, key, value):
        return super().__setitem__(key, [value] * 2)

d = DoubleDict(a=1)
d["b"] = 2
print(d)

outputs

{'a': 1, 'b': [2, 2]}

We see that the user-defined __setitem__ method is correctly used when setting a new value, but is ignored at initialization.

solution

subclass collections.UserDict instead, which is intended to avoid these subtle issues

[matthewturk]

seems fine