Update to the latest stable release. Changes from previous release are:

Added

• os_sorted, os_sort_keygen, and os_sort_key to better support sorting like the file browser on the current operating system
• Support for Python 3.9

Changed

• Treate None like NaN internally to avoid TypeError
• No longer fail tests every time a new Python version is released

Fixed

• Various typos, missing figures, and out-of-date information in the "How it works"
• Fix typo in CHANGELOG
• Updated "How it works" to account for Pandas updates