v0.12.1

This is an incremental release that is a recommended upgrade for all
users. It addresses a handful of bugs / regressions in v0.12.0 and adds
several features and enhancements to the new objects interface.

• Added the objects.Text mark (#3051).
• Added the objects.Dash mark (#3074).
• Added the objects.Perc stat (#3063).
• Added the objects.Count stat (#3086).
• The objects.Band and objects.Range marks will now cover the full
  extent of the data if min / max variables are not explicitly assigned or
  added in a transform (#3056).
• The objects.Jitter move now applies a small amount of jitter by
  default (#3066).
• Axes with a objects.Nominal scale now appear like categorical axes
  in classic seaborn, with fixed margins, no grid, and an inverted y
  axis (#3069).
• The objects.Continuous.label method now accepts base=None to override the default formatter
  with a log transform (#3087).
• Marks that sort along the orient axis (e.g. objects.Line) now use a
  stable algorithm (#3064).
• Added a label parameter to pointplot,
  which addresses a regression in 0.12.0 when pointplot is passed to
  FacetGrid (#3016).
• Fixed a bug that caused an exception when more than two layers with
  the same mappings were added to objects.Plot (#3055).
• Made objects.PolyFit robust to missing data (#3010).
• Fixed a bug in objects.Plot that occurred when data assigned to the
  orient coordinate had zero variance (#3084).
• Fixed a regression in kdeplot where passing cmap for an unfilled bivariate plot would
  raise an exception (#3065).
• Addressed a performance regression in lineplot with a large number
  of unique x values (#3081).
• Seaborn no longer contains doctest-style examples, simplifying the
  testing infrastructure (#3034).