Set --parallel-no-spinner if in CI

[ziima]

What's the problem this feature will solve?

--parallel-no-spinner is fairly required for CI runs to prevent the spinner from messing the output. It seems reasonable to set the flag on by default if CI is detected.

Describe the solution you'd like

Set the --parallel-no-spinner default based on is_ci() utility.

Alternative Solutions

No alternatives.

Caveats

There's no --parallel-spinner which would force the spinner if it was disabled by the CI detection. TOX_PARALLEL_NO_SPINNER=0 should still work in those (hopefully rare) cases.

[gaborbernat]

A pull request in this regard would be accepted.