Docs: Temporarily pin docutils to fix bullets in sphinx_rtd_theme

[hugovk]

Bullet points are missing at https://pillow.readthedocs.io/en/stable/releasenotes/8.3.2.html and on other pages.

This is due to sphinx_rtd_theme not working properly with latest docutils 0.17.

sphinx_rtd_theme issue and fix:

• rendering wrong in docutils 0.17 (unordered list, maybe more?) readthedocs/sphinx_rtd_theme#1115
• Add support for docutils 0.17 readthedocs/sphinx_rtd_theme#1185

Build with 0.16:

Build with 0.17:

Let's pin to docutils 0.16 until the sphinx_rtd_theme fix is out.

[hugovk]

Fixed build: https://pillow--5704.org.readthedocs.build/en/5704/releasenotes/8.3.2.html

[radarhere]

Thanks