gnu: vigra, notmuch: Use the Python3 Sphinx variant.

* gnu/packages/image.scm (vigra)[native-inputs]: Change PYTHON2-SPHINX to
PYTHON-SPHINX.
* gnu/packages/mail.scm (notmuch)[native-inputs]: Likewise.

gnu/packages/image.scm

@@ -893,7 +893,7 @@ graphics image formats like PNG, BMP, JPEG, TIFF and others.")
    (native-inputs
     `(("doxygen" ,doxygen)
       ("python2-nose" ,python2-nose)
-      ("python2-sphinx" ,python2-sphinx)))
+      ("sphinx" ,python-sphinx)))
    (arguments
     `(#:test-target "check"
       #:phases

gnu/packages/mail.scm

@@ -897,7 +897,7 @@ invoking @command{notifymuch} from the post-new hook.")
        ("pkg-config" ,pkg-config)
        ("python" ,python-2)
        ("python-docutils" ,python2-docutils)
-       ("python-sphinx" ,python2-sphinx)
+       ("sphinx" ,python-sphinx)
 
        ;; The following are required for tests only.
        ("which" ,which)