v2.3.0

Minor release with these changes:

• Consistently translate Python enums to values (#198)
• Reverted the removal of the _value postfix (#234)
• Use built-in dict as OrderedDict for Py 3.7+ (#248)
• Fixed crash when sending a query with type definitions (#257)
• Added py.typed file to support type checking (#259)
• A newer Promise version is required (#260)
• Various minor fixes