OpenImageIO 2.1.17.0

Release 2.1.17 (1 Jul 2020) -- compared to 2.1.16

• Build: Use the discovered python binary name, to address the Fedora
  retriction that you must use "python2" or "python3" by name. #2598
• Docs: ImageBufAlgo::nonzero_region had been inadvertently left out of the
  Python chapter.
• Improve RLA reader's ability to detect corrupt or non-RLA files, which
  fixes crashes you could get from trying to read non-image files. #2600
• Support for building against Qt 5.15. (Note: Qt support is only needed
  for the "iv" viewer.) #2605
• Fixes to support LibRaw 0.20 (which is currently in beta3). Note that this
  will make it incompatible with 0.20 beta1 and beta2, due to a fixed typo
  of a struct field in one of the LibRaw's headers. #2613
• oiioversion.h: fix typo that left the OIIO_VERSION_RELEASE_TYPE symbol
  undefined. #2616