Allow creating annotation arrows w/ default props.

[anntzer]

annotate(..., arrowprops={}) now uses an arrow with the default arrow
properties, rather than no arrow. Note that this matches the docstring
of annotate, which indicates that "arrowprops, if not None, is a
dictionary of line properties (see matplotlib.lines.Line2D) for the
arrow that connects annotation to the point."

Comes down to checking for None instead of falsiness of arrowprops.

[tacaswell]

👍 to merging this change.

Although it is not a documented API change, I think this should get an api_changes entry noting that the code has been brought inline with the documentation.

[anntzer]

done