Adding a dilation command to the above example:
import numpy as np
import pylab as plt
from skimage import io, color, filter, exposure, morphology
img = color.rgb2gray(io.imread('7AEJTuA.jpg'))
threshold = filter.threshold_otsu(img)
img_bw = img < threshold
img_bw_thick = morphology.dilation(img_bw, morphology.disk(6))
plt.gray()
f, (ax0, ax1) = plt.subplots(1, 2)
ax0.imshow(img)
ax1.imshow(img_bw_thick)
plt.show()
I see the following image: