This fixes #392.

Previously under Python 3 (probably prior to 3.6, when dicts became
ordered by default) or under Python 2 with ``-R`` or
``PYTHONHASHSEED=random`` the results were undefined because dict
iteration order is undefined.

The value being sorted is complex and defined by
pkg_resources.Distribution:

```    @property
    def hashcmp(self):
        return (
            self.parsed_version,
            self.precedence,
            self.key,
            _remove_md5_fragment(self.location),
            self.py_version or '',
            self.platform or '',
        )

    def __hash__(self):
        return hash(self.hashcmp)

    def __lt__(self, other):
        return self.hashcmp < other.hashcmp
...
```