Share SciTools/cf-units#557 `tox.ini` improvements via templating?

[scitools-ci]

SciTools/cf-units#557 (by @trim21) includes changes to tox.ini. This file is not currently templated, but its parent directory suggests it may be a good candidate. Please either:

• Action this issue with a pull request adding a template file to templates/.
• Close this issue if the file is not a good candidate for templating.

[HGWright]

This change is specific to CF-Units so no change required.