make enum singletons into attributes of the enum class

[cosmicexplorer]

See #7226 (comment). After that PR is merged, we should do something like:

diff --git a/src/python/pants/util/objects.py b/src/python/pants/util/objects.py
index e22e4b9..8208d0f 100644
--- a/src/python/pants/util/objects.py
+++ b/src/python/pants/util/objects.py
@@ -6,7 +6,7 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 
 import sys
 from abc import abstractmethod
-from builtins import object, zip
+from builtins import object, zip, str
 from collections import OrderedDict, namedtuple
 
 from future.utils import PY2
@@ -208,7 +208,16 @@ def enum(*args):
 
   Any enum subclass can be constructed with its create() classmethod. This method will use the first
   element of `all_values` as the default value, but enum classes can override this behavior by
-  setting `default_value` in the class body.
+  setting `default_value` in the class body.
+
+  If `all_values` contains only strings, then each variant is made into an attribute on the
+  generated enum class object. This allows code such as the following:
+
+  class MyResult(enum(['success', 'failure'])):
+    pass
+
+  MyResult.success # The same as: MyResult(success)
+  MyResult.failure # The same as: MyResult(failure)
 
   :param string field_name: A string used as the field for the datatype. This positional argument is
                             optional, and defaults to 'value'. Note that `enum()` does not yet
@@ -332,6 +342,11 @@ def enum(*args):
       """
       return [cls.create(variant_value) for variant_value in cls.allowed_values]
 
+  if all(isinstance(v, str) for v in all_values_realized):
+    for variant in ChoiceDatatype.iterate_enum_variants():
+      setattr(ChoiceDatatype, ChoiceDatatype._get_value(variant), variant)
+
   return ChoiceDatatype