proto-plus >= 1.25 needed for python 3.13

[parthea]

The setup.py file should be updated to require proto-plus >= 1.25 for python_version >=3.13 due to googleapis/proto-plus-python#490

See release notes here: https://github.com/googleapis/proto-plus-python/releases/tag/v1.25.0

Add "proto-plus >= 1.25.0, <2.0.0dev; python_version>='3.13'", after the following line

https://github.com/googleapis/python-spanner/blob/d3c646479bdc2ad96f68ed7d4703188dcdd76395/setup.py#L44

As an example, see googleapis/python-api-core#740

[chalmerlowe]

This issue was moved from the following repo as part of the monorepo migration: googleapis/python-spanner