Fix tox.ini for tox v4

[kkrrz]

Fixes tox error that space-separated variables are not allowed in tox 4

py310-cov: failed with pass_env values cannot contain whitespace, use comma to have multiple values in a single line, invalid values found 'PYTEST_ADDOPTS TERM'
py310-cov: FAIL code 1 (0.00 seconds)
evaluation failed :( (1.00 seconds)
Error: Process completed with exit code 1.