python-build-system: Fix 'get-python-version'.

* guix/build/python-build-system.scm (get-python-version): Rewrite to handle
  multiple-digit version number components.

guix/build/python-build-system.scm

@@ -55,7 +55,10 @@ (define* (check #:key tests? test-target #:allow-other-keys)
     #t))
 
 (define (get-python-version python)
-  (string-take (string-take-right python 5) 3))
+  (let* ((version     (last (string-split python #\-)))
+         (components  (string-split version #\.))
+         (major+minor (take components 2)))
+    (string-join major+minor ".")))
 
 (define* (install #:key outputs inputs (configure-flags '())
                   #:allow-other-keys)