fix: include license and tests in sdist

[nightcityblade]

Fixes #660

Summary

• explicitly include LICENSE.txt in PDM source distributions
• include the complete tests/ tree for downstream packagers without adding it to wheels

Testing

• uv build (sdist and wheel)
• verified the archived license is byte-identical to LICENSE.txt
• verified the sdist contains 92 test entries and the wheel contains none
• uv run --project .. --frozen python runtests.py (from tests/)
• uv run --frozen flake8 docxtpl tests setup.py --count --max-line-length=127 --show-source --statistics

[CAOShurong]

Reviewed the exact PR head 055d03f627d41af111eff55ef5238d1363e7ab63 independently.

• The published PyPI 0.20.2 sdist has 0 license entries and 0 test entries.
• A build from this PR contains the byte-identical LICENSE.txt (SHA-256 06eb1c44...) and 92 test entries in the sdist.
• The wheel contains the license under .dist-info/licenses/ and contains no tests.
• The 36 non-CLI test scripts passed in a clean wheel-installed Python 3.13 environment; the module CLI path also passed separately and produced a 13,419-byte DOCX.
• The repository's flake8 command and git diff --check both passed.

The latest pdm-backend also happens to include these files implicitly when rebuilding the base commit today, but declaring source-includes explicitly makes the intended sdist contents robust instead of depending on backend-default drift. This looks correct and focused to me.

OpenAI Codex assisted with the reproduction and verification. I reviewed the diff and the observed results before approving.