gnu: Add python-ipyparallel-bootstrap.

* gnu/packages/python-xyz.scm (python-ipyparallel): Morph into...
(python-ipyparallel-bootstrap): ... this, moving native inputs to a new
definition of...
(python-ipyparallel): ... this.

gnu/packages/python-xyz.scm

@@ -8782,9 +8782,11 @@ away.")
 (define-public python2-ipython-genutils
   (package-with-python2 python-ipython-genutils))
 
-(define-public python-ipyparallel
+;;; Variant used to break a cycle with python-ipykernel.
+(define-public python-ipyparallel-bootstrap
+  (hidden-package
    (package
-    (name "python-ipyparallel")
+     (name "python-ipyparallel-bootstrap")
      (version "6.2.4")
      (source
       (origin
@@ -8795,30 +8797,22 @@ away.")
           "0rf0dbpxf5z82bw8lsjj45r3wdd4wc74anz4wiiaf2rbjqlb1ivn"))))
      (build-system python-build-system)
      (arguments
-     `(#:tests? #f ; RuntimeError: IO Loop failed to start
+      (list
-       #:phases
+       #:tests? #f
-       (modify-phases %standard-phases
+       #:phases #~(modify-phases %standard-phases
-         (add-before 'check 'prepare-for-tests
+                    ;; The python-ipykernel is normally propagated but is
-           (lambda _
+                    ;; removed from this package to break the cycle.
-             (setenv "HOME" (getcwd))
+                    (delete 'sanity-check))))
-             #t)))))
      (propagated-inputs
       (list python-dateutil
             python-decorator
-           python-ipykernel
+            ;; python-ipykernel is omitted here to avoid a cycle.
             python-ipython
             python-ipython-genutils
-           python-jupyter-client
+            python-jupyter-client-bootstrap
             python-pyzmq
             python-tornado
             python-traitlets))
-    (native-inputs
-     (list python-ipython
-           python-mock
-           python-nose
-           python-pytest
-           python-pytest-cov
-           python-testpath))
      (home-page "https://ipython.org/")
      (synopsis "Interactive Parallel Computing with IPython")
      (description
@@ -8830,7 +8824,26 @@ CLI scripts:
 @item ipcontroller - start a scheduler
 @item ipengine - start an engine
 @end enumerate")
-    (license license:bsd-3)))
+     (license license:bsd-3))))
+
+(define-public python-ipyparallel
+  (package
+    (inherit python-ipyparallel-bootstrap)
+    (name "python-ipyparallel")
+    (arguments (list #:tests? #t))
+    (native-inputs
+     (list python-ipython
+           python-mock
+           python-nose
+           python-pytest
+           python-pytest-cov
+           python-testpath))
+    (propagated-inputs
+     (modify-inputs (package-propagated-inputs python-ipyparallel-bootstrap)
+       (replace "python-jupyter-client-bootstrap" python-jupyter-client)
+       (append python-ipykernel)))
+    (properties (alist-delete 'hidden? (package-properties
+                                        python-ipyparallel-bootstrap)))))
 
 (define-public python-ipython-cluster-helper
   (package
@@ -9119,10 +9132,16 @@ installing @code{kernelspec}s for use with Jupyter frontends.")
       (package
         (inherit parent)
         (name "python-ipykernel-bootstrap")
+        (arguments (list #:tests? #f
+                         ;; The package should normally propagate ipykernel,
+                         ;; left out here to break the cycle.
+                         #:phases #~(modify-phases %standard-phases
+                                      (delete 'sanity-check))))
+        (native-inputs '())
         (propagated-inputs
-          `(("python-jupyter-client" ,python-jupyter-client-bootstrap)
+         (modify-inputs (package-propagated-inputs parent)
-            ,@(fold alist-delete (package-propagated-inputs parent)
+           (replace "python-jupyter-client" python-jupyter-client-bootstrap)
-                    '("python-jupyter-client"))))))))
+           (append python-ipyparallel-bootstrap)))))))
 
 (define-public python-pari-jupyter
   (package