attempt to fix readthedocs

Summary:
Something's wrong with recommonmark/CommonMark/six, let's see if this fixes it.

https://readthedocs.org/projects/pytorch3d/builds/21292632/

```
  File "/home/docs/checkouts/readthedocs.org/user_builds/pytorch3d/envs/latest/lib/python3.11/site-packages/sphinx/config.py", line 368, in eval_config_file
    execfile_(filename, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pytorch3d/envs/latest/lib/python3.11/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pytorch3d/checkouts/latest/docs/conf.py", line 25, in <module>
    from recommonmark.parser import CommonMarkParser
  File "/home/docs/checkouts/readthedocs.org/user_builds/pytorch3d/envs/latest/lib/python3.11/site-packages/recommonmark/parser.py", line 6, in <module>
    from CommonMark import DocParser, HTMLRenderer
  File "/home/docs/checkouts/readthedocs.org/user_builds/pytorch3d/envs/latest/lib/python3.11/site-packages/CommonMark/__init__.py", line 3, in <module>
    from CommonMark.CommonMark import HTMLRenderer
  File "/home/docs/checkouts/readthedocs.org/user_builds/pytorch3d/envs/latest/lib/python3.11/site-packages/CommonMark/CommonMark.py", line 18, in <module>
    HTMLunescape = html.parser.HTMLParser().unescape
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'HTMLParser' object has no attribute 'unescape'
```

Reviewed By: shapovalov

Differential Revision: D47471545

fbshipit-source-id: 48e121e20da535b3cc46b6bd2393d28869067b8b

docs/requirements.txt

 docutils>=0.14
 Sphinx>=1.7
-recommonmark==0.4.0
+recommonmark
 sphinx_rtd_theme
 sphinx_markdown_tables
 numpy