📝 Document when {tty} was added

[webknjaz]

I used the {tty} substitution in a tox.ini of one project and the minimum version set was ancient (~1.5) so I wanted to find out when it was introduced and both places where I'd expect this info weren't helpful.

I tracked it down to v3.4.0 via git blame, of course. But I think it'd be useful to actually have it in the Sphinx docs. So here you go. No change note because it's unnecessary here.

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword
  in message body
• added news fragment in changelog folder
  • fragment name: <issue number>.<type>.rst for example (588.bugfix.rst)
  • <type> is must be one of bugfix, feature, deprecation,breaking, doc, misc
  • if PR has no issue: consider creating one first or change it to the PR number after creating the PR
  • "sign" fragment with "by :user:<your username>"
  • please use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files - by :user:superuser."
  • also see examples
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[asottile]