chore(python): Add Python 3.12

[gcf-owl-bot]

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BEGIN_COMMIT_OVERRIDE
feat: add support for Python 3.12
END_COMMIT_OVERRIDE

[parthea]

Wait for cl/585116344

[holtskinner]

Looks like a module removed in 3.12 is used by the hocr_spec dependency. It's only used in Testing, not the actual library.

https://docs.python.org/3.11/library/imp.html

_________________ ERROR collecting tests/unit/test_document.py _________________
ImportError while importing test module '/home/runner/work/python-documentai-toolbox/python-documentai-toolbox/tests/unit/test_document.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/opt/hostedtoolcache/Python/3.12.0/x64/lib/python3.12/importlib/__init__.py:90: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_document.py:36: in <module>
    from hocr_spec import HocrValidator
.nox/unit-3-12/lib/python3.12/site-packages/hocr_spec/__init__.py:6: in <module>
    from .spec import HocrSpec
.nox/unit-3-12/lib/python3.12/site-packages/hocr_spec/spec.py:3: in <module>
    from future import standard_library
.nox/unit-3-12/lib/python3.12/site-packages/future/standard_library/__init__.py:65: in <module>
    import imp
E   ModuleNotFoundError: No module named 'imp'