Bump markdown from 2.6.7 to 3.8.1 in /protolib

Bumps [markdown](https://github.com/Python-Markdown/markdown) from 2.6.7 to 3.8.1.
- [Release notes](https://github.com/Python-Markdown/markdown/releases)
- [Changelog](https://github.com/Python-Markdown/markdown/blob/master/docs/changelog.md)
- [Commits](https://github.com/Python-Markdown/markdown/commits/3.8.1)

---
updated-dependencies:
- dependency-name: markdown
  dependency-version: 3.8.1
  dependency-type: direct:production
...

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Author: dependabot[bot]

protolib/requirements.txt

@@ -1,4 +1,4 @@
-markdown==2.6.7
+markdown==3.8.1
 beautifulsoup4==4.5.1
 Faker==0.7.3
 python-slugify==1.2.1