TST/CLN: Replace all tmpdir / os.path operations in tests with pathlib.Path

[brendan-ward]

Resolves #400

This removes all instances of the legacy pytest tmpdir fixture, and uses the tmp_path (a pathlib.Path object) fixture instead.

Modernized the various path operations in the tests to use methods on the pathlib.Path object.

Removes unused win32.py test file; we have actual tests against windows and this is no longer used.

Note: I updated the benchmarks to use tmp_path and verified that most of them still run (some fail with missing test files), but we're not actively maintaining these.

[theroggy]

Nice cleanup!

[jorisvandenbossche]

Looks good!

There are a few cases where we still pass str(filename) to some read function and where the str(..) can be removed now (after your previous PR to uniformize the path handling)