REL: v0.6.0

This is a major release recommended for all users.

- Added direct dependence on imageio (PR 392); imageio-ffmpeg is optional (PR 358)
- Added compatibility with Python 3.10 and 3.11 (PR 383, PR 399)
- Ended support for Python 2 (PR 408)
- PIMS reader objects now support the ``dtype`` and ``shape`` attributes from the
  standard numpy array interface (PR 254)
- Fixed frame numbering bug for PyAV (PR 370)
- Fixed handling of nonstandard datetime tags in TIFFs (PR 375)
- Updated documentation (PR 376, PR 390)
- Fixed and improved automated testing and development (PR 380, PR 383,
  PR 399, PR 391, PR 407, PR 410)
- Removed use of deprecated skimage.external.tifffile (PR 362)