gnu: python-pygments: Update to 2.7.4 [fixes CVE-2021-27291].

* gnu/packages/python-xyz.scm (python-pygments/fixed): New variable.
(python-pygments)[replacement]: Graft.

gnu/packages/python-xyz.scm

@@ -3701,6 +3701,7 @@ text styles of documentation.")
        (sha256
         (base32
          "05mps9r966r3dpqw6zrs1nlwjdf5y4960hl9m7abwb3qyfnarwyc"))))
+    (replacement python-pygments/fixed)
     (build-system python-build-system)
     (arguments
      ;; FIXME: Tests require sphinx, which depends on this.
@@ -3712,6 +3713,17 @@ text styles of documentation.")
     (license license:bsd-2)
     (properties `((python2-variant . ,(delay python2-pygments))))))
 
+(define python-pygments/fixed
+  (package/inherit python-pygments
+    (version "2.7.4")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (pypi-uri "Pygments" version))
+       (sha256
+        (base32
+         "0dy35ry5qa8dpklk4fkj9kfpw2qb4mh5ha9866kw30wf96dx0jfz"))))))
+
 ;; Pygments 2.6 and later does not support Python 2.
 (define-public python2-pygments
   (let ((base (package-with-python2 (strip-python2-variant python-pygments))))