Version 0.13.0

PlottableProtocol provides a way to plot in different libraries, and easy
access to common quantities. This is expected to be the final release for
Python 2, and mostly equivalent in API to 1.0.

User changes

• Support for PlottableProtocol. You can now access .values(), .counts(),
  and .variances() on all storages; used by plotting libraries. .kind describes
  the Kind of the histogram (bh.Kind.COUNT or bh.Kind.MEAN). .options has
  been renamed to .traits, and a few more useful traits were added, like
  .discrete. Most other portions of the Protocol were already present. #476
• Removed deprecated .rank on histograms (since 0.8). Use .ndim instead. #505
• Supports converting user histogram objects that provide a
  _to_boost_histogram_ method. #483
• A view=True parameter must now be passed to get a View instead of a standard
  NumPy values arrray from to_numpy(). #498

Bug fixes

• Added additional support for typing, fixing a couple of rare Python 2 bugs in the process #493.
• The resulting histogram from bh.numpy.* functions is now reducible #508

Developer changes

• Use GitHub Actions for ARM compiling #474
• Apple Silicon support (since 0.12) #495
• Support compiling with C++17 #502
• Rename NPY_NUM_BUILD_JOBS to CMAKE_BUILD_PARALLEL_LEVEL for consistency
  with other Scikit-HEP projects. #502