CLI: don't color if stdout redirected.

[hjoliver]

These changes close #4552 and supersede #4559

Current colour output disabling is a bit broken.

On master, this correctly generates colour output in the terminal:

$ cylc cat-log test 

But these cases (for example) get unnecessary color-code pollution:

$ cylc cat-log test | vim -   # IN YOUR FACE emacs!!! (can't read from stdin)
$ cylc cat-log test > log

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.

• Appropriate tests are included (unit and/or functional).
• Already covered by existing tests.
• Does not need tests (why?).

• Appropriate change log entry included.

• No change log entry required (why? e.g. invisible to users).

• No documentation update required.

[wxtim]

• Makes sense as a change.
• Works with the test

  $ cylc validate simplest > log
  $ cat -A log
  # On master:
  ^[[32mValid for cylc-8.0rc1.dev^[[0m^[[0m$\n^[[0m
  # On branch
  Valid for cylc-8.0rc1.dev

RE Testing - I spent a good deal of time yesterday considering how to test this, and struggled. I'm happy to manually test this.

Question: I'm happy with manually testing things, but might it be worth having a document somewhere listing and explaining manual tests.

[oliver-sanders]

Yeah, I think this is sensible, why would you want to pipe colour?

Also reading from stdin is so dammed useful.

[oliver-sanders]

@wxtim this was waiting on the unchecked bullets in the description.

[hjoliver]

No worries, I'll post a follow-up.