Fix nightly cron ctypes enum failure (pantsbuild#7249)

### Problem

Resolves pantsbuild#7247. `ToolchainVariant('gnu')` does not in fact `== 'gnu'`.

### Solution

- Use `.resolve_for_enum_variant()` instead of comparing with equality in that one failing test (I missed this in pantsbuild#7226, I fixed the instance earlier in the file though).
- Raise an error when trying to use `==` on an enum to avoid this from happening again.
- Note that in Python 3 it appears that `__hash__` must be explicitly implemented whenever `__eq__` is overridden, and this appears undocumented.

### Result

The nightly cron job should be fixed, and enums are now a little more difficult to screw up.

# Open Questions
It's a little unclear why this didn't fail in CI -- either the test was cached, or some but not all travis osx images are provisioned with the correct dylib, causing a nondeterministic error, or something else?

src/python/pants/util/objects.py

@@ -132,13 +132,16 @@ def __eq__(self, other):
     def __ne__(self, other):
       return not (self == other)
 
+    # NB: in Python 3, whenever __eq__ is overridden, __hash__() must also be
+    # explicitly implemented, otherwise Python will raise "unhashable type". See
+    # https://docs.python.org/3/reference/datamodel.html#object.__hash__.
     def __hash__(self):
       return super(DataType, self).__hash__()
 
     # NB: As datatype is not iterable, we need to override both __iter__ and all of the
     # namedtuple methods that expect self to be iterable.
     def __iter__(self):
-      raise TypeError("'{}' object is not iterable".format(type(self).__name__))
+      raise self.make_type_error("datatype object is not iterable")
 
     def _super_iter(self):
       return super(DataType, self).__iter__()
@@ -281,6 +284,20 @@ def __new__(cls, value):
       """
       return cls.create(value)
 
+    # TODO: figure out if this will always trigger on primitives like strings, and what situations
+    # won't call this __eq__ (and therefore won't raise like we want).
+    def __eq__(self, other):
+      """Redefine equality to raise to nudge people to use static pattern matching."""
+      raise self.make_type_error(
+        "enum equality is defined to be an error -- use .resolve_for_enum_variant() instead!")
+    # Redefine the canary so datatype __new__ doesn't raise.
+    __eq__._eq_override_canary = None
+
+    # NB: as noted in datatype(), __hash__ must be explicitly implemented whenever __eq__ is
+    # overridden. See https://docs.python.org/3/reference/datamodel.html#object.__hash__.
+    def __hash__(self):
+      return super(ChoiceDatatype, self).__hash__()
+
     @classmethod
     def create(cls, *args, **kwargs):
       """Create an instance of this enum, using the default value if specified.

tests/python/pants_test/backend/python/tasks/native/test_ctypes_integration.py

@@ -183,7 +183,10 @@ def test_ctypes_third_party_integration(self, toolchain_variant):
     # TODO(#6848): this fails when run with gcc on osx as it requires gcc's libstdc++.so.6.dylib to
     # be available on the runtime library path.
     attempt_pants_run = Platform.create().resolve_for_enum_variant({
-      'darwin': toolchain_variant != 'gnu',
+      'darwin': toolchain_variant.resolve_for_enum_variant({
+        'gnu': False,
+        'llvm': True,
+      }),
       'linux': True,
     })
     if attempt_pants_run:

tests/python/pants_test/util/test_objects.py

@@ -741,6 +741,26 @@ def test_enum_instance_creation_errors(self):
     with self.assertRaisesRegexp(EnumVariantSelectionError, expected_rx_falsy_value):
       SomeEnum('')
 
+  def test_enum_comparison_fails(self):
+    enum_instance = SomeEnum(1)
+    rx_str = re.escape("enum equality is defined to be an error")
+    with self.assertRaisesRegexp(TypeCheckError, rx_str):
+      enum_instance == enum_instance
+    with self.assertRaisesRegexp(TypeCheckError, rx_str):
+      enum_instance != enum_instance
+    # Test that comparison also fails against another type.
+    with self.assertRaisesRegexp(TypeCheckError, rx_str):
+      enum_instance == 1
+    with self.assertRaisesRegexp(TypeCheckError, rx_str):
+      1 == enum_instance
+
+    class StrEnum(enum(['a'])): pass
+    enum_instance = StrEnum('a')
+    with self.assertRaisesRegexp(TypeCheckError, rx_str):
+      enum_instance == 'a'
+    with self.assertRaisesRegexp(TypeCheckError, rx_str):
+      'a' == enum_instance
+
   def test_enum_resolve_variant(self):
     one_enum_instance = SomeEnum(1)
     two_enum_instance = SomeEnum(2)