Remove old pythons and leverage @ operator

[bnavigator]

We should finally let go of Python 2. It will allow us to use the matmul operator @ and make code more readable.

As NumPy has deprecated Python 3.6 support a while ago (NEP 29), Conda recently dropped it and it will also reach end-of-life in the mainline Python this month, let's also stop testing Python 3.6.

There is an important difference to consider when replacing np.dot or ndarray.dot with @:

• Multiplication by scalars is not allowed, use * instead.
• Stacks of matrices are broadcast together as if the matrices were elements, respecting the signature (n,k),(k,m)->(n,m):

[coveralls]

Coverage increased (+0.005%) to 92.721% when pulling de94123 on bnavigator:remove-old-python into c2f6f8a on python-control:master.

[sawyerbfuller]

I was really excited when python got the matrix mult operator. really cleans up the code. So it sounds reasonable to me if nobody can give a good reason to keep python 2 around.