fix: Properly quote file paths with spaces

[effigies]

Summary

This PR aims to ensure that we handle files/directories with spaces correctly. Unfortunately, there are a number of interfaces that separately handle the problem by including quotes in their argstrs, some with single quotes, some with double quotes. We have to assume that there are downstream tools that do the same.

This therefore narrowly addresses the case where the argstr is a plain-as-dirt %s and the trait is either a File/Directory or a list of such things.

Fixes #3604.

List of changes proposed in this PR (pull-request)

• Add a space to the base test directory.
• Specifically test a file name with a space in it.

Unrelated changes that I don't want to open separate PRs for

• Add lxml to dependencies so the package will install and import.
• Fix a bbregister bug that I'm not sure how I came to notice, where the isinstance check had been copied from the previous if block but not updated to match the new context.
• Add NO_ET to the tox environments to improve startup times for testing.