Pin markdown==3.3.7 to prevent mkautodoc loading failure

[florimondmanca]

This PR gets CI back in shape as there is an issue with mkautodoc when the latest Markdown version is installed:

Python 3.10.0 (default, Nov 28 2021, 23:05:33) [GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import mkautodoc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/florimond/dev/fairness/httpx/venv/lib/python3.10/site-packages/mkautodoc/__init__.py", line 1, in <module>
    from .extension import MKAutoDocExtension, makeExtension
  File "/home/florimond/dev/fairness/httpx/venv/lib/python3.10/site-packages/mkautodoc/extension.py", line 4, in <module>
    from markdown.util import etree
ImportError: cannot import name 'etree' from 'markdown.util' (/home/florimond/dev/fairness/httpx/venv/lib/python3.10/site-packages/markdown/util.py)
>>> 

https://python-markdown.github.io/change_log/release-3.4/

Previously deprecated objects have been removed

Deprecated Object	Replacement Object
markdown.util.etree	xml.etree.ElementTree

[Kludex]

Ah, that was it. 😅

[florimondmanca]

@tomchristie I opened tomchristie/mkautodoc#33 to fix the underlying issue.

(Would we want to move this project to Encode so anyone in encode/maintainers can help maintain it?)