Actually, it’s quite easy: instead of the number of bins you can give a list with the bin boundaries. They can be unequally distributed, too:
plt.hist(data, bins=[0, 10, 20, 30, 40, 50, 100])
If you just want them equally distributed, you can simply use range:
plt.hist(data, bins=range(min(data), max(data) + binwidth, binwidth))
Added to original answer
The above line works for data filled with integers only. As macrocosme points out, for floats you can use:
import numpy as np
plt.hist(data, bins=np.arange(min(data), max(data) + binwidth, binwidth))