Add third-party license notices for bundled schemas and graphs util

[tefra]

Closes #1220

Changes

• Add the W3C Software and Document Short Notice to the vendored schemas copied from W3C: mathml3*.xsd (5 files), xlink.xsd, xml.xsd. Each notice gives the source URL, copyright year and changes made.
• Add xsdata/schemas/LICENSE.W3C, which lists every W3C-derived file with its source and changes, plus the full W3C Software and Document License text.
• xsi.xsd is not copied from W3C. It is a minimal schema written for xsdata, and a comment now says so.
• xsdata/utils/graphs.py: credit the original MIT-licensed sources, ActiveState recipe 578507 (Copyright 2013 Mark Dickinson) and mypy (Jukka Lehtosalo and contributors, Dropbox, Inc.).
• Add THIRD_PARTY_LICENSES.md with the MIT text, and list it and LICENSE.W3C in license-files, so wheels ship them under dist-info/licenses/. The project license stays MIT.

Notes

• The MathML and XLink files are byte-identical to upstream apart from line endings. xml.xsd has the documentation annotations and the stylesheet instruction removed.
• soapenc.xsd already carried its full W3C notice, so it is unchanged.

Testing

• pytest: all tests pass.
• Built the wheel and checked that dist-info/licenses/ contains LICENSE, THIRD_PARTY_LICENSES.md and xsdata/schemas/LICENSE.W3C.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.97%. Comparing base (8138cb7) to head (46f8497).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1238   +/-   ##
=======================================
  Coverage   99.97%   99.97%           
=======================================
  Files         117      117           
  Lines        9414     9414           
  Branches     1447     1447           
=======================================
  Hits         9412     9412           
  Misses          1        1           
  Partials        1        1           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud