colors has no field TwoSlopeNorm

Hi everyone,

It appears that "colors" function imported from matplotlib does not have "Normalize" or "TwoSlopeNorm" field.
Any idea about this?

`using PyPlot`
`vmin, vmax = -1, 1`
`norm = colors.TwoSlopeNorm(vmin=vmin, vcenter=0., vmax=vmax)`

ERROR: type #colors has no field TwoSlopeNorm

Anyways, my goal is to set the colorbar zero at center with symmetric color range.

```
pcolormesh(data, cmap="RdBu_r", norm = norm, shading="auto")
Colorbar()
```