test: turn PL deprecation warnings into errors

[timmysilv]

confirmed locally that it works, and it won't be raised if the warning is caught/suppressed in a test

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c12cbe8) 99.65% compared to head (3558476) 99.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4823      +/-   ##
==========================================
- Coverage   99.65%   99.64%   -0.01%     
==========================================
  Files         381      381              
  Lines       34346    34086     -260     
==========================================
- Hits        34226    33965     -261     
- Misses        120      121       +1     

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[albi3ro]

So I get:

============================================================== short test summary info ===============================================================
ERROR tests/drawer/test_draw.py - pennylane.PennyLaneDeprecationWarning: Decorating a QNode with @transform_fn(**transform_kwargs) has been deprecated and will be removed in a fut...
ERROR tests/transforms/test_batch_transform.py - pennylane.PennyLaneDeprecationWarning: Use of `batch_transform` to create a custom transform is deprecated. Instead switch to using the new qml.t...
ERROR tests/transforms/test_op_transform.py - pennylane.PennyLaneDeprecationWarning: Use of `op_transform` to create a custom transform is deprecated. Instead switch to using the new qml.tran...
ERROR tests/transforms/test_qfunc_transform.py - pennylane.PennyLaneDeprecationWarning: Use of `qfunc_transform` to create a custom transform is deprecated. Instead switch to using the new qml.t...
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 4 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

which is weird that this happens locally for me but not in the CI.

Especially if you aren't seeing any problems ... 🤷

[timmysilv]

nice find, turns out I was seeing it locally too! I only tested a particular case locally. The error comes up during test collection, and we don't see it in CI because we specify the -p no:warnings option, which suppresses the early-loading of warnings. I added the option to pytest.ini, so now running tests locally should work fine.

[albi3ro]

I am wondering if we should instead explicitly silence the warnings that occur during collection, but I'm ok with this solution too.