1.17.3: pixel_width and height support, minor bug and docfixes

• add pixel_height and pixel_width, for libsixel support or general curiosity, it actually works on most of the terminals I tested! This wasn't the case several years ago, closes #155
• bugfix "pickleability" of FormatOtherString, by implementing __getnewargs__, with tests.
• docfix some confusion about (y, x), (row, column), etc. There were some backwards things, and it's pretty annoying that get_location() doesn't match location(), we document why -- to retain API compatibility with blessings!
• rewrite the messages of our TypeError's, hopefully these are easier to understand.