Migrate `tests/bin/test_jsonpath.py` to use pytest

[kurtmckee]

Also, the empty __init__.py in the same directory has been deleted; pytest doesn't rely on test suites being importable modules.

Also, these tools were run against the test file:

• pyupgrade --py37-plus
• black
• isort

If this merges, I'll migrate the rest of the test suite to use pytest, too.

[michaelmior]

LGTM, thanks!