[python] Migrate PyPaimon packaging to pyproject.toml

[aisk]

Purpose

Migrate PyPaimon packaging metadata from setup.py and requirements files to pyproject.toml.

This PR keeps setuptools as the build backend, raises the minimum Python version to 3.7, and moves development dependencies into the dev dependency group.

Closes #8049.

Changes

• Add paimon-python/pyproject.toml with project metadata, runtime dependencies, optional dependencies, scripts, package discovery, and the dev dependency group.
• Remove setup.py, dev/requirements.txt, and dev/requirements-dev.txt.
• Update PyPaimon README and docs to use python3 -m build --sdist.
• Update MANIFEST.in now that requirements files are removed.

Tests

• python3 -c "import tomllib; tomllib.load(open('pyproject.toml','rb')); print('ok')"
• UV_CACHE_DIR=/tmp/uv-cache uv build --sdist

[aisk]

I found that there are some test cases failed on Python3.7, which are better to fix in another PR. I'll mark this as draft and will continue the work if the CI cased fixed.