Symfit

n = np.array([5, 3, 8, 15, 30])
sigma_t = sigma / np.sqrt(n)

We now define our model
y = Variable()
g = Parameter()
t_model = (2 * y / g)**0.5

fit = Fit(t_model, y_data, t_data, sigma=sigma_t)
fit_result = fit.execute()

First stable release of Symbolic Fitting!