fix: Install weasyprint as a xml2rfc extra

[kesara]

Since xml2rfc v3.23.0, WeasyPrint version is pinned.

This will also enforce the pydyf version¹.

Footnotes

1. https://github.com/ietf-tools/xml2rfc/blob/main/setup.cfg#L59-L60 ↩

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (c7f6bde) to head (a0ac21b).
Report is 42 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7845      +/-   ##
==========================================
+ Coverage   88.78%   88.80%   +0.01%     
==========================================
  Files         296      303       +7     
  Lines       41320    41427     +107     
==========================================
+ Hits        36687    36790     +103     
- Misses       4633     4637       +4     

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

[rjsparks]

I thought about dropping the explicit declaration of weasyprint from datatracker's requirements, relying on inheriting it from xml2rfc (to avoid some future update of the versions we require, perhaps coming in by automation, from breaking against the pin in xml2rfc), but making it obvious we have the dependency (for more than xml2rfc) is worth that maintenance burden.