Always expand file paths using / when calling LaTeX

[naveen521kk]

Using \ isn't supported by LaTeX and simply errors out
saying it can't find the file. In this commit, the path is
converted to POSIX like path (ie. use /) before calling
LaTeX.

Overview: What does this pull request change?

This fixes LaTeX erroring out on Windows.
Fixes #3060

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[behackl]

So there was an issue after all -- thanks for spotting this. I'll take a look at the other recently submitted PRs, perhaps there are some more bugfixes that should be included -- but otherwise yeah, it does call for 0.17.1.