Unit Step Response of an Integrator Desinging a very basic 1st order integrator with the continous transfer function:

$H(s) = -1 / (0.001 s)$
[No zeros, one pole at origin]

I was expecting the unit step response to be positive-slope.
However in my Python code I get a negative slope. 

num = (0 , -1)
den = (0.001 , 0)
Hs = sig.lti(num,den)
t, s = sig.step(Hs)
plt.plot(t,s)


Provides:
Your integrator has a negative sign:

$$H(s)=-\frac{1000}{s}\tag{1}$$

That's why its step response also has a negative sign:

$$y(t)=-1000\int_{-\infty}^tu(\tau)d\tau=-1000\cdot t\cdot u(t)\tag{2}$$

where $u(t)$ is the unit step.