Fix json WindowsPath crash (#8662)
* Add check for WindowsPath in to_json_string
On Windows, os.path.join returns a WindowsPath. to_json_string does not convert this from a WindowsPath to a string. Added check for WindowsPath to to_json_saveable.
* Remove extraneous convert to string in test_check_path_types (tests/others/test_config.py)
* Fix style issues in tests/others/test_config.py
* Add unit test to test_config.py to verify that PosixPath and WindowsPath (depending on system) both work when converted to JSON
* Remove distinction between PosixPath and WindowsPath in ConfigMixIn.to_json_string(). Conditional now tests for Path, and uses Path.as_posix() to convert to string.
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Co-authored-by: Vincent Dovydaitis <vincedovy@gmail.com>
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
