Why do we squint when tasting very sour things?
Short answer
Adverse stimuli in general are accompanied by contraction of facial muscles. It is part of our repertoire of emotional expressions. 

Background
A disgusted expression and the expression of pain are both accompanied by squinting. Disgusting stimuli include your example of a sour taste, but also include other adverse tastes (bitter) or visual stimuli such as a gross-looking objects. In other words, squinting is likely a common way of expressing adverse stimuli. So the "sour look" may not be easily explained on its own. Instead, it is part of a huge emotional expressive repertoire that makes us human beings. To be able to read these expressions allows us to be empathetic towards others. People with autism and Asperger's syndrome may not be able to do so and are disadvantaged in their social behaviors. 

To illustrate the commonalities between sour (#1), disgust (#2) and pain (#3) I would like to share the following web finds.


Sour taste. Source: Getty Images

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Disgust. Source: Berkeley Emotional IQ test


Pain. Source: Berkeley Emotional IQ test  

PS: I am not a psychologist - Consider my contribution as an educated guess.