textcode markup: fix SyntaxWarning in regex

[gotmax23]

Tasks

• [x] Reviewed contribution guidelines
• [x] PR is descriptively titled 📑 and links the original issue above
• [x] Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR Run tests locally to check for errors.
• [x] Commits are in uniquely-named feature branch and has no merge conflicts 📁
• [x] Updated documentation pages (if applicable)
• [x] Updated CHANGELOG.rst (if applicable)