bpo-41816: add StrEnum (GH-22337)

`StrEnum` ensures that its members were already strings, or intended to
be strings.

Lib/enum.py

@@ -4,7 +4,7 @@ from types import MappingProxyType, DynamicClassAttribute
 
 __all__ = [
         'EnumMeta',
-        'Enum', 'IntEnum', 'Flag', 'IntFlag',
+        'Enum', 'IntEnum', 'StrEnum', 'Flag', 'IntFlag',
         'auto', 'unique',
         ]
 
@@ -688,7 +688,35 @@ class Enum(metaclass=EnumMeta):
 
 
 class IntEnum(int, Enum):
-    """Enum where members are also (and must be) ints"""
+    """
+    Enum where members are also (and must be) ints
+    """
+
+
+class StrEnum(str, Enum):
+    """
+    Enum where members are also (and must be) strings
+    """
+
+    def __new__(cls, *values):
+        if len(values) > 3:
+            raise TypeError('too many arguments for str(): %r' % (values, ))
+        if len(values) == 1:
+            # it must be a string
+            if not isinstance(values[0], str):
+                raise TypeError('%r is not a string' % (values[0], ))
+        if len(values) > 1:
+            # check that encoding argument is a string
+            if not isinstance(values[1], str):
+                raise TypeError('encoding must be a string, not %r' % (values[1], ))
+            if len(values) > 2:
+                # check that errors argument is a string
+                if not isinstance(values[2], str):
+                    raise TypeError('errors must be a string, not %r' % (values[2], ))
+        value = str(*values)
+        member = str.__new__(cls, value)
+        member._value_ = value
+        return member
 
 
 def _reduce_ex_by_name(self, proto):