This is the common Sphinx theme for all PyPA projects.
It is derived from the Python docs theme, since PyPA maintains portions of the default Python developer experience (specifically, the software installation and distribution tools bootstrapped by the ensurepip module).
To use the theme either clone it directly with git, or else install it into your docs build environment via pip:
pip install git+https://github.com/pypa/pypa-docs-theme.git#egg=pypa-docs-theme