What does this implement/fix? Explain your changes.

This determines the value for Runtime.UCS at runtime instead of using compile-time defines, removing the need for separate builds of Python.Runtime.

Does this close any currently open issues?

N/A

Any other comments?

PyUnicode_GetMax is deprecated, but preserved for compatibility.

Checklist

Check all those that are applicable and complete.

• Make sure to include one or more tests for your change
• If an enhancement PR, please create docs and at best an example
• Add yourself to AUTHORS
• Updated the CHANGELOG