Update idna requirement from >=3.15 to >=3.18

Updates the requirements on [idna](https://github.com/kjd/idna) to permit the latest version.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.md)
- [Commits](kjd/idna@v3.15...v3.18)

---
updated-dependencies:
- dependency-name: idna
  dependency-version: '3.18'
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -4,4 +4,4 @@ mkdocstrings>=0.24.0
 mkdocstrings-python>=1.7.4
 markdown-exec>=1.10.3
 pymdown-extensions>=10.21.3
-idna>=3.15  # CVE-2026-45409 (re-fix of CVE-2024-3651): idna.encode() DoS on crafted long inputs
+idna>=3.18  # CVE-2026-45409 (re-fix of CVE-2024-3651): idna.encode() DoS on crafted long inputs