BUG: fix two bugs in yt.visualization.eps_writer

[neutrinoceros]

PR Summary

Noticed that eps_writer didn't have tests so I ran an example from the documentation and was able to find a trivial but blocking bug.
While testing this I also fixed another bug where save_fig didn't accept Path objects.

PR Checklist

[N/A] New features are documented, with docstrings and narrative docs

• Adds a test for any bugs fixed. Adds tests for new features.

[neutrinoceros]

Turns out testing this in CI is hard. I'll get back to it.

[jwise77]

Thanks for creating this test and fixing the bug. All of the changes you've made so far look good. I can review any other future changes in this PR.

[neutrinoceros]

So I think the reason why CI was failing is that the "tex" executable isn't available on the virtual machines we test with. Unfortunately this means that my test is doomed to never run automatically, which limits greatly its value. On the other hand it seems clear that this module isn't heavily used currently since no body seemed to have noticed it was broken, and it's still better to have a reference test than no tests.

On a broader level, I'm thinking this module likely has applicability far beyond yt and it would probably make sense to migrate it to its own project ? it would also help making sure it's properly tested.
And even before that, I understand that you wrote this functionality out of frustration with matplotlib's own rasterising capabilities, any idea if it got any better since then ?
This discussion goes well beyond the scope of this PR though, maybe we should open a dedicated issue.

[neutrinoceros]

What's important to review this PR is to make sure my test works at least locally.

[jwise77]

This all looks good. Thanks again. I agree that it'll be easier to test this module if it's an extension. I'll make an issue and work on it soon.

[matthewturk]

@meeseeksdev backport to yt-4.0.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout yt-4.0.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 b1e3e289f2efd0c35a993b6c520ef284b9a6df28

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #3496: BUG: fix two bugs in yt.visualization.eps_writer'

4. Push to a named branch :

git push YOURFORK yt-4.0.x:auto-backport-of-pr-3496-on-yt-4.0.x

5. Create a PR against branch yt-4.0.x, I would have named this PR:

"Backport PR #3496 on branch yt-4.0.x (BUG: fix two bugs in yt.visualization.eps_writer)"

And apply the correct labels and milestones.

Congratulation you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove Still Needs Manual Backport label once the PR gets merged.

If these instruction are inaccurate, feel free to suggest an improvement.

[neutrinoceros]

@meeseeksdev backport to yt-4.0.x