Fixes bug in compiler modified_time handling

[davehughes]

I had the same issue as #211 (and possibly #210) and this seems to be a reasonable solution. When I dug into the issue, I found:

• The compiler passes absolute file paths for infile and outfile when trying to determine modification times.
• The modified_time method on Django's file storage objects uses the storage's path() method as part of its determination
• path() expects a storage-relative name, which is at odds with passing absolute file paths
• path() doesn't find the named file, and falls back to the default StaticFilesStorage handling, which for some reason looks for the absolute file in the static directory of the first app in my INSTALLED_APPS, notices that the requested file is outside of that directory, and raises a SuspiciousOperation error.

FYI, I'm relying on the `django.contrib.staticfiles.finders.AppDirectoriesFinder' finder, which may be part of the cause of this issue.

[davehughes]

I also tried writing a test or two to reproduce this behavior, but I'm fairly new to tox and wasn't able to come up with a good way to exercise the compiler side of things (LESS, in my case). Do you have any pointers for installing Node/NPM and using them to test compilation with pipeline?

[cyberdelia]

Thanks, I'm afraid we might be missing some corner case here, dealing with app directories is hard.

No good pointer, unfortunately, except that it's a mess 😑
And testing for one compiler is pointless, they all work differently 😢

[cyberdelia]

Oh wait! You're totally right. I'll merge this and make a releaseI
If you still want to make some test case, please do, but don't try to run less or something similar, you can create your own dummy compiler that mock what you expect. And don't let tox intoxicate you.
Thank you, you rock! ✨