build(deps): update markdown requirement in /_build

Updates the requirements on [markdown](https://github.com/Python-Markdown/markdown) to permit the latest version.
- [Changelog](https://github.com/Python-Markdown/markdown/blob/master/docs/changelog.md)
- [Commits](Python-Markdown/markdown@3.2.1...3.5)

---
updated-dependencies:
- dependency-name: markdown
  dependency-type: direct:production
...

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

_build/requirements.txt

@@ -3,7 +3,7 @@ click==8.1.7
 ghp-import==2.1.0
 importlib-metadata==6.8.0
 Jinja2==3.1.2
-Markdown>=3.2.1,<3.5
+Markdown>=3.2.1,<3.6
 MarkupSafe==2.1.3
 mergedeep==1.3.4
 mkdocs==1.5.3