An MCVE for a bug in tox

pyproject.toml

[build-system]
requires = [
    "read_version ~= 0.1.0",
    "setuptools >= 34.4.0",
    "wheel"
]
build-backend = "setuptools.build_meta"

setup.py

from read_version import read_version
from setuptools   import setup

setup(
    name='tox-bug',
    version=read_version('tox_bug.py'),
    py_modules=['tox_bug'],
)

tox.ini

[tox]
envlist = py35,py36,py37
skip_missing_interpreters = True
isolated_build = True

[testenv]
usedevelop = True
commands =

tox_bug.py

__version__ = '0.1.0'