DOC: Use square figsize for pie plots
First commit closes https://github.com/pydata/pandas/issues/8308

second closes https://github.com/pydata/pandas/issues/8312 aside from using mpl 1.4 to build the docs.

I can post any examples if you want.

@TomAugspurger ?
 Squashed down to one commit. @jorisvandenbossche these should take care of everything for the doc build other than using mpl 1.4 to fix the gridlines.
