v0.21.0rc1

This is a major release from 0.20.3 and includes a number of API
changes, deprecations, new features, enhancements, and performance
improvements along with a large number of bug fixes. We recommend that
all users upgrade to this version.

Highlights include:

• Integration with Apache Parquet,
  including a new top-level read_parquet function and
  DataFrame.to_parquet method, see here
• New user-facing pandas.api.types.CategoricalDtype for specifying
  categoricals independent of the data, see
  here
• The behavior of sum and prod on all-NaN Series/DataFrames is now
  consistent and no longer depends on whether
  bottleneck is installed,
  see here
• Compatibility fixes for pypy, see here

Check the API Changes whatsnew; and deprecations; before updating.