Fix Bloch sphere distortion

[BoxiLi]

Matplotlib stopped to stretch the plot to fit it in a square box from 3.3.0. We do it manually. See https://matplotlib.org/3.3.0/users/whats_new.html#d-axes-improvements

Description
To reproduce

import numpy as np
from qutip import Bloch

b = Bloch()
xp = [np.cos(th) for th in np.linspace(0, 2*np.pi, 20)]
yp = [np.sin(th) for th in np.linspace(0, 2*np.pi, 20)]
zp = np.zeros(20)
pnts = [xp, yp, zp]
b.add_points(pnts)
b.show()

before:

after:

Related issues or PRs
fix #1385

Changelog
Fix Bloch sphere distortion

[jakelishman]

If only this had come an hour sooner! Making a new release of code is pretty straightforwards now, though, so we can fold this and some other bits and bobs into a patch release in a week or two's time.

[BoxiLi]

😆 Well, technically we still need to wait for the test.

Glad to see 4.6.0 online!

[coveralls]

Coverage increased (+0.002%) to 64.118% when pulling f635fb8 on BoxiLi:bloch-sphere-ratio into 22f8be4 on qutip:master.