contrib: add tox data directory to gitignore

[SnoopJ]

Description

The tox configuration in contrib/ will store its working data (virtual environments, logs, scratch space) in contrib/.tox, which shows up in git status. This changeset adds that location to .gitignore (a brand new contrib/.gitignore for managing contrib files specifically) to avoid untracked file clutter for users (read as: @dgw) who opt into the tox workflow.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches