Add windows and macos python 3.6 unit test runners; make filesystem operations safer on Windows

[rtibbles]

Summary

Adds windows and macos to unit tests, running only on Python 3.6
Removes use of NamedTemporaryFile as all our usages required opening the file by reference to the filepath which is not supported on Windows (see https://docs.python.org/3/library/tempfile.html#tempfile.NamedTemporaryFile). Instead uses mkstemp which is safer.

Reviewer guidance

Do tests pass?
Does caching function correctly on new platforms?

Contributor Checklist

PR process:

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Testing:

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

Reviewer Checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md