Replace OrderedSet with dict

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.

setuptools/dist.py

@@ -11,7 +11,6 @@
 from typing import TYPE_CHECKING, MutableMapping
 
 from more_itertools import partition, unique_everseen
-from ordered_set import OrderedSet
 from packaging.markers import InvalidMarker, Marker
 from packaging.specifiers import InvalidSpecifier, SpecifierSet
 from packaging.version import Version
@@ -251,7 +250,7 @@ class Distribution(_Distribution):
     _DISTUTILS_UNSUPPORTED_METADATA = {
         'long_description_content_type': lambda: None,
         'project_urls': dict,
-        'provides_extras': OrderedSet,
+        'provides_extras': dict,  # behaves like an ordered set
         'license_file': lambda: None,
         'license_files': lambda: None,
         'install_requires': list,
@@ -349,7 +348,7 @@ def _finalize_requires(self):
                 # Setuptools allows a weird "<name>:<env markers> syntax for extras
                 extra = extra.split(':')[0]
                 if extra:
-                    self.metadata.provides_extras.add(extra)
+                    self.metadata.provides_extras.setdefault(extra)
 
     def _normalize_requires(self):
         """Make sure requirement-related attributes exist and are normalized"""

setuptools/tests/config/test_setupcfg.py

@@ -675,7 +675,7 @@ def test_extras_require(self, tmpdir):
                 'pdf': ['ReportLab>=1.2', 'RXP'],
                 'rest': ['docutils>=0.3', 'pack==1.1,==1.3'],
             }
-            assert dist.metadata.provides_extras == set(['pdf', 'rest'])
+            assert set(dist.metadata.provides_extras) == {'pdf', 'rest'}
 
     @pytest.mark.parametrize(
         "config",

setuptools/tests/test_core_metadata.py

@@ -188,7 +188,7 @@ def test_read_metadata(name, attrs):
         ('requires', dist_class.get_requires),
         ('classifiers', dist_class.get_classifiers),
         ('project_urls', lambda s: getattr(s, 'project_urls', {})),
-        ('provides_extras', lambda s: getattr(s, 'provides_extras', set())),
+        ('provides_extras', lambda s: getattr(s, 'provides_extras', {})),
     ]
 
     for attr, getter in tested_attrs:

setuptools/tests/test_dist.py

@@ -77,12 +77,12 @@ def test_provides_extras_deterministic_order():
     extras['b'] = ['bar']
     attrs = dict(extras_require=extras)
     dist = Distribution(attrs)
-    assert dist.metadata.provides_extras == ['a', 'b']
+    assert list(dist.metadata.provides_extras) == ['a', 'b']
     attrs['extras_require'] = collections.OrderedDict(
         reversed(list(attrs['extras_require'].items()))
     )
     dist = Distribution(attrs)
-    assert dist.metadata.provides_extras == ['b', 'a']
+    assert list(dist.metadata.provides_extras) == ['b', 'a']
 
 
 CHECK_PACKAGE_DATA_TESTS = (