Replace ordered_set with dict

[abravalheri]

Summary of changes

This is a follow up of the discussion on #4457 (review).

Since Python 3.7, dict preserve insertion order.
This means that dict is a capable implementation of an ordered set,
despite the method names not lining up completely.

We can effectively use dict to obtain the same functionality that would be implemented by ordered_set and therefore we can remove the extra dependency.

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

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Optionally, we could have something like

# setuptools/_ordered_set.py

from collections.abc import MutableSet

class ordered_set(dict, MutableSet):
    add = dict.setdefault

    def discard(self, item):
        self.pop(item, None)

But that would be mostly cosmetic, since we can simply use the dict methods directly and obtain the same effects...

[Avasam]

Searching for "ordered" in python files, there's a few bits of logic that still assume dicts are unordered. It's getting late here, but I could open a PR in parallel of yours with all those I found.

[Avasam]

Line 313 still contains a reference to "ordered-set==3.1.1" (it's in an example, but may as well remove it)

[jaraco]

I'm +0.

[jaraco]

Thanks! LGTM.