Add Python 3.13 support (#1048)

Fixes https://github.com/NVIDIA/Model-Optimizer/issues/217

## Summary

- Bump `requires-python` from `>=3.10,<3.13` to `>=3.10,<3.14` to
formally include Python 3.13
- Add explicit Python 3.10–3.13 PyPI classifiers for better
discoverability
- Add `py313` to tox CPU unit test and partial-install environment
matrices
- Add Python 3.10–3.13 to the `multi-py` CI matrix in `unit_tests.yml`

## Background

Python 3.13 was previously excluded by the `<3.13` upper bound. Testing
in a related repo with `--ignore-requires-python` confirmed that the
library installs and runs correctly under Python 3.13. This PR lifts the
restriction and wires up CI to verify it going forward.

## Test plan

- [ ] CI `multi-py` job passes on `py313-torch210-tf_latest-unit`
- [ ] `tox -e py313-torch210-tf_latest-unit` passes locally (requires
Python 3.13 installed)
- [ ] `tox -e py313-partial-unit-torch` passes locally
- [ ] No regressions on existing Python 3.10/3.11/3.12 matrix jobs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Extended Python support: minimum remains 3.10; added official support
up through 3.13 (upper bound advanced accordingly).
* **Tests**
* CI and test matrix expanded to include experimental Python 3.13
coverage.
* **Documentation**
* Installation docs and changelog updated to reflect Python 3.13
support.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Ivan Basov <ibasov@nvidia.com>
Signed-off-by: Ivan Basov <5455484+ivanbasov@users.noreply.github.com>
Signed-off-by: Keval Morabia <28916987+kevalmorabia97@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Keval Morabia <28916987+kevalmorabia97@users.noreply.github.com>

docs/source/getting_started/_installation_for_Linux.rst

@@ -12,7 +12,7 @@ Latest Model Optimizer (``nvidia-modelopt``) currently has the following system
 +-------------------------+-----------------------------+
 | Architecture            |  x86_64, aarch64 (SBSA)     |
 +-------------------------+-----------------------------+
-| Python                  |  >=3.10,<3.13               |
+| Python                  |  >=3.10,<3.14               |
 +-------------------------+-----------------------------+
 | CUDA                    |  12.x, 13.x                 |
 +-------------------------+-----------------------------+