Use isolated build for tox

[bhrutledge]

This resolves an error when running any tox environment:

% tox -e mypy
GLOB sdist-make: /Users/bhrutledge/Dev/readme_renderer/setup.py
...
running check
Traceback (most recent call last):
...
  File "/Users/bhrutledge/Dev/readme_renderer/readme_renderer/rst.py", line 19, in <module>
    from docutils.core import publish_parts
ModuleNotFoundError: No module named 'docutils'

% tox --version
3.24.5 imported from /Users/bhrutledge/.local/pipx/venvs/tox/lib/python3.8/site-packages/tox/__init__.py

See https://tox.wiki/en/latest/config.html#conf-isolated_build