Update so3.py

[zmk5]

When using the liegroups lib I run into the following SyntaxWarning when using the so3.py file

liegroups/numpy/so3.py:83: SyntaxWarning: "is" with a literal. Did you mean "=="?
  elif ordering is 'wxyz':

I've changed the literal with two equal signs for this file to prevent the warning. I will look through some of the other files to see if I can find any more needing to be changed.