Pin pygments<2.20 to fix docs build (pymdownx filename=None crash)

Pygments 2.20.0 added html.escape() in HtmlFormatter.__init__ that
crashes when pymdown-extensions passes filename=None. Pin to 2.19.x
until pymdown-extensions releases a compatible fix.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rustyconover

pyproject.toml

@@ -173,6 +173,7 @@ docs = [
     "mkdocstrings[python]>=0.27",
     "mkdocs-section-index>=0.3",
     "mkdocs-d2-plugin>=1.4",
+    "pygments>=2.19,<2.20",
 ]
 dev = [
     "aioresponses>=0.7",