IO updates

[bmcfee]

Fixes #415
Fixes #413

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This PR adds support for pathlib.Path objects in all mir_eval.io loaders. (Easy fix, just adding a case to the _open context manager's type check.)

I've added one unit test that hits load_delimited with either string or Path input. I think this is sufficient since: A) almost all other loaders run through load_delimited, and B) those that don't use the same _open context manager anyway.

Since I was in .io anyway, I added a deprecation on load_wav.

Assuming this passes CI, I think it should be good to merge.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Flag	Coverage Δ
unittests	85.67% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
mir_eval/io.py	92.04% <100.00%> (+0.09%)	⬆️

[bmcfee]

Rewrote this a bit following #413 (comment) ; I'm not wild about raising an IOError from a TypeError, for many reasons. (It's obtuse, provides no additional information, and adds complexity. It also doesn't do anything useful if open throws a different kind of error.)

I've done it here so that we preserve the type of exception raised in case of failure, as changing it would constitute API breakage and I'd rather not do that in a minor release (as this is slated for). But going forward (0.9), we should rip out this indirection and just let open() throw whatever exception it likes back up the stack.