bump package version

[noahho]

Motivation and Context

---

Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

---

How Has This Been Tested?

---

Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• A entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

---

[Copilot]

Pull Request Overview

This PR bumps the package version from 2.1.2 to 2.1.3 in the pyproject.toml configuration file.

• Updates the version number in the project metadata

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[gemini-code-assist]

Code Review

This pull request bumps the package version to 2.1.3. My review confirms the version change but highlights a necessary accompanying update to the CHANGELOG.md file. To ensure users are aware of the changes in this new release, the changelog should be updated to reflect version 2.1.3.

[gemini-code-assist]

While bumping the version number is correct, this change should be accompanied by an update to CHANGELOG.md to document the release. The current [Unreleased] section should be renamed to [2.1.3] and include the release date. A new, empty [Unreleased] section should be added at the top for future changes.

For example:

# Changelog
...
## [Unreleased]

## [2.1.3] - YYYY-MM-DD

### Added
- Added several new finetuned model checkpoints. ([#462](https://github.com/PriorLabs/TabPFN/pull/462))

### Bug Fixes
- Fixed a validation error that occurred when a `.env` file contained settings from other applications. ([#446](https://github.com/PriorLabs/TabPFN/pull/446))
- Fixed a crash on PyTorch versions older than 2.5 by correctly detecting Grouped-Query Attention (GQA) support. ([#438](https://github.com/PriorLabs/TabPFN/pull/438))
...

This is crucial for maintaining a clear and informative history of changes for users of the package.