Bump version to 2.0.4 and make constants.py the single source of truth

[Saegl]

The project currently has version drift:

• __init__.py reports version 2.0.0.
• constants.py and pyproject.toml report version 2.0.3.
• GitHub Releases already contains a 2.0.4 release.

This mismatch caused the PyPI publishing workflow to fail:

https://github.com/mailersend/mailersend-python/actions/runs/29732208568/job/88319075241

This PR makes the version in constants.py the single source of truth:

• __init__.py now imports the version from constants.py.
• pyproject.toml now uses dynamic = ["version"].

More information about this approach:

• https://packaging.python.org/en/latest/discussions/single-source-version/#single-source-version
• https://hatch.pypa.io/1.17/version/

Follow-up required

Merging this PR alone will not fix the existing PyPI release. A maintainer will also need to update the Git tag and rerun the build-n-publish workflow:

git tag -f v2.0.4 <bump-commit-sha>
git push --force origin v2.0.4

Alternatively, a new v2.0.5 release can be created if force-updating the existing tag is not desirable.

[hfbmb]

@igorhrcek Could you please take a look at this issue? This problem is currently preventing MailerSend Async v2.0.4 from being released to PyPI.

Please fix the issue and release a new version to PyPI once the fix is in place. It would be great to get this resolved so we can proceed with the updated release. Thanks!