Inline a copy of _PyUnicode_Copy

[movermeyer]

What are you trying to accomplish?

Silences compiler errors in Python 3.13. Fixes #61.

python/cpython#106382 moved the _PyUnicode API functions elsewhere.

What approach did you choose and why?

Instead of backporting the current Python 3.13 datetimemodule.c code, I simply inlined the _PyUnicode_Copy method. Much easier.

I also changed the example pip install command in the README to include the ; python_version < '3.11' version specifier, to hopefully encourage correct usage.

What should reviewers focus on?

🤷

The impact of these changes

Silences compiler errors in Python 3.13.

While no one should bother using backports.datetime_fromisoformat with Python 3.13, people still do (presumably since they don't know to use ; python_version < '3.11' in their dependencies).

Testing

Build once again work in Python 3.13:

env_313/bin/python -m build