fix: relax lxml dependency constraint

[nightcityblade]

Summary

Relaxes the lxml dependency constraint so Crawl4AI can resolve with packages that require lxml 6.x, such as Scrapling.

Fixes #2019

List of files changed and why

• pyproject.toml - allow lxml>=5.3,<7 instead of restricting to the 5.x series.
• requirements.txt - keep the development requirements aligned with project metadata.
• uv.lock - update the package metadata constraint recorded for Crawl4AI.

How Has This Been Tested?

• uv lock
• uv lock --project <temp project requiring crawl4ai @ file:///tmp/crawl4ai and scrapling==0.4.9> --dry-run
• uv run python - <<'PY' ... import crawl4ai ... PY

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes