Replace ast.Ellipsis with ast.Constant

[fabaff]

Fixes issue with Python 3.14

[....]
       > self = <pscript.commonast.NativeAstConverter object at 0x7ffff48143d0>
       > n = Constant(value='a', kind=None)
       >
       >     def _convert_index_like(self, n):
       >         c = self._convert
       > >       if isinstance(n, (ast.Slice, ast.Index, ast.ExtSlice, ast.Ellipsis)):
       >                                                               ^^^^^^^^^^^^
       > E       AttributeError: module 'ast' has no attribute 'Ellipsis'
       >
       > pscript/commonast.py:1035: AttributeError
       > =========================== short test summary info ============================
       > FAILED tests/test_commonast.py::test_consistent_ast1 - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser1.py::TestExpressions::test_indexing_and_slicing - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser1.py::TestExpressions::test_assignments - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser1.py::TestExpressions::test_dict_literals - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser1.py::TestExpressions::test_funcion_call - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser1.py::TestExpressions::test_delete - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser2.py::TestConrolFlow::test_for - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser2.py::TestConrolFlow::test_listcomp_regressions - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser3.py::TestOtherBuiltins::test_repr - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > FAILED tests/test_parser3.py::TestDictMethods::test_copy - AttributeError: module 'ast' has no attribute 'Ellipsis'
       > ================ 10 failed, 213 passed, 1 deselected in 31.37s =================
``

[almarklein]

Could you add Py314 to CI by duplicating plus adjusting these lines:

pscript/.github/workflows/ci.yml

Lines 81 to 83 in e6060e6

	- name: Linux py 3.13
	os: ubuntu-latest
	pyversion: '3.13'

Also please run ruff format from the repo roor to fix the linting errors (Ruff sometimes updates, requiring reformatting).

[almarklein]

mmm, I guess we'd still want Ellipses in that tuple for Python < 3.14. Maybe it'd be better to have something like?

if python_version < (3, 14):
   Ellipses = ast.Ellipses
else:
   Ellipses = ast.Constant