Avoid modifying path placeholders created by editable installs

[jeremy-hiatt]

The setuptools implementation of editable installs will insert a placeholder entry into sys.path as part of its magic to register its custom import mechanism.

These are not real filesystem paths and as such should not be rewritten to absolute paths.

This is a proposed fix for #1028 .

Checklist:

• Make sure to include reasonable tests for your change if necessary

I couldn't figure out how to test this specific behavior, but happy to add one given guidance on how to modify sys.path in the test environment.

• We use towncrier for changelog management, so please add a news file

Added.

[RonnyPfannschmidt]

This looks good, any idea how to test this sanely

[jeremy-hiatt]

This looks good, any idea how to test this sanely

@RonnyPfannschmidt , I tried a bit longer in response to your comment but I still can't figure out how to set up the test so it runs with a custom sys.path element. I found the monkeypatch.syspath_prepend() method but it doesn't seem to work here. Would you like me to keep trying or are you able to merge as-is?

[jeremy-hiatt]

Hi @RonnyPfannschmidt , I think I found a reasonably straightforward way to create a test, and I added this commit to the PR. If there's nothing else, I'd appreciate it if you're able to merge!