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I've had difficulty getting Sage to handle Python functions, like your sqstim, in differential equations. Sage wants to execute sqstim(t) immediately when it is called in the desolve_odeint command.

Here is an option using numpy and scipy.

import numpy as np
from scipy.integrate import odeint

def eqns(y,t):
    V=y[0]
    w=y[1]
    dydt = [1/100.*(-w+(V^3-V)+sqstim(t)), V-0.2*w]
    return(dydt)

tspan=np.linspace(0,100,1000)

sol = odeint(eqns, [0.1,0.1], tspan)

tV = zip(tspan,[s[0] for s in sol])
tw = zip(tspan,[s[1] for s in sol])

Then, you can plot easily using Sage commands.

line(tV,color='red')

and

line(tw)