gnu: python-nbformat: Update to 5.1.3.

* gnu/packages/python-xyz.scm (python-nbformat): Update to 5.1.3.
[arguments]: Remove to enable tests.
[native-inputs]: Add python-pytest.
[properties]: Add python2-variant.
(python2-nbformat): Keep at version 4.4.0; override version and source field.

gnu/packages/python-xyz.scm

@@ -10190,29 +10190,42 @@ Debian-related files, such as:
 (define-public python-nbformat
   (package
     (name "python-nbformat")
-    (version "4.4.0")
+    (version "5.1.3")
     (source
      (origin
        (method url-fetch)
        (uri (pypi-uri "nbformat" version))
        (sha256
         (base32
-         "00nlf08h8yc4q73nphfvfhxrcnilaqanb8z0mdy6nxk0vzq4wjgp"))))
+         "1j6idwsw59cslsssvlkg2bkfpvd6ri7kghbp14jwcw87sy57h5mm"))))
     (build-system python-build-system)
-    (arguments `(#:tests? #f)) ; no test target
     (propagated-inputs
      `(("python-ipython-genutils" ,python-ipython-genutils)
        ("python-jsonschema" ,python-jsonschema)
        ("python-jupyter-core" ,python-jupyter-core)
        ("python-traitlets" ,python-traitlets)))
+    (native-inputs
+     `(("python-pytest" ,python-pytest)))
     (home-page "https://jupyter.org")
     (synopsis "Jupyter Notebook format")
     (description "This package provides the reference implementation of the
 Jupyter Notebook format and Python APIs for working with notebooks.")
+    (properties `((python2-variant . ,(delay python2-nbformat))))
     (license license:bsd-3)))
 
 (define-public python2-nbformat
-  (package-with-python2 python-nbformat))
+  (let ((parent (package-with-python2
+                 (strip-python2-variant python-nbformat))))
+    (package
+      (inherit parent)
+      (version "4.4.0")
+      (source
+       (origin
+         (method url-fetch)
+         (uri (pypi-uri "nbformat" version))
+         (sha256
+          (base32
+           "00nlf08h8yc4q73nphfvfhxrcnilaqanb8z0mdy6nxk0vzq4wjgp")))))))
 
 (define-public python-bleach
   (package