yolk.metadata.get_metadata only gets the value of the last repeating header type

It's common for python packages to list multiple classifiers in `setup.py`. `yolk` itself does so too: https://github.com/cakebread/yolk/blob/develop/setup.py.
But, as the list of key-value pairs from `email.message_from_string` is copied into a dictionary, only the last key-value pair with the same key is returned in the metadata dictionary.
