Bloch sphere deformed for matplotlib >= 3.3.0

[steffen-micdev]

Describe the bug
When plotting the Bloch sphere with qutip and matplotlib >=3.3.0 the sphere appears deformed in z-direction.
This is a result of a change in matplotlib Axes3D: https://matplotlib.org/3.3.0/users/whats_new.html#d-axes-improvements

To Reproduce

import qutip
b = qutip.Bloch()
b.show()

Work around

1. downgrade to matplotlib 3.2.2
   or
2. set aspect ratio manually

import qutip
b = qutip.Bloch()
b.show()
if hasattr(b.axes, 'set_box_aspect'):
    b.axes.set_box_aspect([1,1,1])

Environment
QuTiP Version: 4.5.2
Numpy Version: 1.19.2
Scipy Version: 1.5.2
Cython Version: 0.29.21
Matplotlib Version: 3.3.2
Python Version: 3.7.9
Number of CPUs: 6
BLAS Info: INTEL MKL
OPENMP Installed: False
INTEL MKL Ext: True
Platform Info: Linux (x86_64)

[paniash]

This is an issue with matplotlib rather than QuTiP. A possible workaround is downgrading to version 3.2.2.

[jakobjakobson13]

The underlying reason is that matplotlib made some changes in the Axes3D module.