v0.22.0 Final

This is a major release from 0.21.1 and includes a single, API-breaking change.
We recommend that all users upgrade to this version after carefully reading the
release note.

The only changes are:

• The sum of an empty or all-NA Series is now 0
• The product of an empty or all-NA Series is now 1
• We've added a min_count parameter to .sum() and .prod() controlling
  the minimum number of valid values for the result to be valid. If fewer than
  min_count non-NA values are present, the result is NA. The default is
  0. To return NaN, the 0.21 behavior, use min_count=1.

See the whatsnew for further explanation of all the places in the library this affects.