Internal handling of axis ticks optimized

No officially documented parts of the API have changed. However, you may have

implicitly relied on a certain behavior.

`Axis.majorTicks` and `Axis.minorTicks` are deprecated

The undocumented attributes `majorTicks` and `minorTicks` of

:class:`matplotlib.axis.Axis` are now explicitly considered private and may be

removed in the future. Please uses :meth:`matplotlib.axis.Axis.get_major_ticks`

and :meth:`matplotlib.axis.Axis.get_minor_ticks` only.

Axis tick lists are not persistent anymore

:meth:`matplotlib.axis.Axis.get_major_ticks` and

:meth:`matplotlib.axis.Axis.get_minor_ticks` have returned a list of ticks

that was valid for the whole lifetime of the axis. Though this behavior was

never guaranteed, users might have stored a reference to this list and

expect it to be updated.

You should not permanently store a reference to the ticks, instead always

fetch an up-to-date list using the above methods when needed.