gnu: python-pyparsing: Use 'modify-phases' syntax.

* gnu/packages/python.scm (python-pyparsing)[arguments]<#:phases>: Use
'modify-phases'.

gnu/packages/python.scm

@@ -3955,11 +3955,9 @@ library, libgit2 implements Git plumbing.")
     (outputs '("out" "doc"))
     (arguments
      `(#:tests? #f ; no test target
-       #:modules ((guix build python-build-system)
-                  (guix build utils))
        #:phases
-       (alist-cons-after
-        'install 'install-doc
+       (modify-phases %standard-phases
+         (add-after 'install 'install-doc
            (lambda* (#:key outputs #:allow-other-keys)
              (let* ((doc (string-append (assoc-ref outputs "doc")
                                         "/share/doc/" ,name "-" ,version))
@@ -3973,8 +3971,8 @@ library, libgit2 implements Git plumbing.")
                          (install-file file tgt))
                        (find-files dir ".*")))
                 (list "docs" "htmldoc" "examples")
-             (list doc html-doc examples))))
-        %standard-phases)))
+                (list doc html-doc examples))
+               #t))))))
     (home-page "http://pyparsing.wikispaces.com")
     (synopsis "Python parsing class library")
     (description