use GridSpec in plt.subplots

[rwirth]

Use a GridSpec object to set up the subplots in plt.subplots. Add an optional parameter gridspec_kw. This enables the user to specify height_ratios and width_ratios to get grids with unequal row/column sizes while still having the convenience that subplots provides.

[tacaswell]

I like this change, but I worry about side effects.

[rwirth]

I haven't done extensive testing on this, but in the simple tests I did (and in the subplots example) there was no noticeable behavior change compared to the old way. Besides, a GridSpec object is also used during the creation of the Subplot when specifying the subplot position as a Matlab-style triple (as it is done now). In the SubplotBase constructor in matplotlib/axes/_subplots.py the triples are converted into SubplotSpecs using GridSpec(rows, cols)[num - 1]. So, the explicit use of GridSpec is not doing anything fundamentally different. I will do some more testing when I find the time for it.