Fix pyzx.draw() causing an error due to missing subplotspec in some matplotlib environments.

[dlyongemallo]

Fixes #222.

[dlyongemallo]

Code review note: This PR should be tested by a variety of users in different environments to ensure the appearance of the diagrams are the same as before. I've manually tested it in different environments on my own machine, and in theory I've set everything which had previously been explicitly set to their existing values, but the call to subplots is now setting the matplotlib.gridspec.SubplotSpec which was previously not set before and could therefore potentially change the figures' appearance in some environments.

[dlyongemallo]

Has anyone had a chance to look at this? I'm pretty sure this fixes the issue in those environments for which missing subplotspec causes a problem, and is a no-op otherwise.

[RazinShaikh]

I tried this and the figures look same to me.

[dlyongemallo]

Can this be merged please?