3.0.3 PyPI tarball is missing CODE_OF_CONDUCT

[mgorny]

Describe the bug
The 3.0.3 .tar.gz found on PyPI does not include the CODE_OF_CONDUCT file. As a result, the documentation fails to build:

Warning, treated as error:                                               
/tmp/portage/dev-python/sphinx-3.0.3/work/Sphinx-3.0.3/doc/code_of_conduct.rst:8:Problems with "include" directive path:                           
InputError: [Errno 2] No such file or directory: 'CODE_OF_CONDUCT'.

To Reproduce
Steps to reproduce the behavior:

wget https://files.pythonhosted.org/packages/12/a9/41a67d6638323b763b284a80b57461fe3557b128efcce892360cd73879ce/Sphinx-3.0.3.tar.gz
tar -xf Sphinx-3.0.3.tar.gz
cd Sphinx-3.0.3
python setup.py build_sphinx

Expected behavior
Surprisingly, I would expect it to build successfully.

Environment info

• OS: Gentoo Linux
• Python version: 3.8.2
• Sphinx version: 3.0.3

Additional context
Curious enough, when I run setup.py sdist locally it includes all the files, so this might be relevant to the environment used to cut the release.