Version 0.9.0

This version was released just before PyHEP 2020. Several important fixes were made,
along with a few new features to better support downstream projects.

User changes

• metadata supported and propagated on Histograms (slots added) #403
• Added dd=True option in to_numpy #406
• Deprecated cpp module removed #402

Developer changes

• Subclasses can override axes generation #401
• [dev] extra now installs pytest #401

Bug fixes

• Fix numpy.histogramdd return structure #406
• Travis deploy multi-arch fixes #399
• Selecting on a bool axes supports 2D+ histograms #398
• Warnings fixed on NumPy 1.19+ [#404][]