gnu: Fix Borg crashes with Python msgpack > 0.5

The Python msgpack library changed its name, but borg (and possibly
other packages) still uses the old name.

Fixes <https://bugs.gnu.org/30662>.

* gnu/packages/python.scm (python-msgpack-transitional): New variable.
* gnu/packages/backup.scm (borg)[inputs]: Use it.

gnu/packages/backup.scm

@@ -547,7 +547,10 @@ (define-public borg
        ("lz4" ,lz4)
        ("openssl" ,openssl)
        ("python-llfuse" ,python-llfuse)
-       ("python-msgpack" ,python-msgpack)
+       ;; The Python msgpack library changed its name so Borg requires this
+       ;; transitional package for now:
+       ;; <https://bugs.gnu.org/30662>
+       ("python-msgpack" ,python-msgpack-transitional)
        ("zstd" ,zstd)))
     (synopsis "Deduplicated, encrypted, authenticated and compressed backups")
     (description "Borg is a deduplicating backup program.  Optionally, it

gnu/packages/python.scm

@@ -5869,6 +5869,24 @@ (define-public python-msgpack
     (home-page "https://pypi.python.org/pypi/msgpack/")
     (license license:asl2.0)))
 
+;; This msgpack library's name changed from "python-msgpack" to "msgpack" with
+;; release 0.5. Some packages like borg still call it by the old name for now.
+;; <https://bugs.gnu.org/30662>
+(define-public python-msgpack-transitional
+  (package
+    (inherit python-msgpack)
+    (arguments
+     (substitute-keyword-arguments (package-arguments python-msgpack)
+       ((#:phases phases)
+        `(modify-phases ,phases
+           (add-after 'unpack 'configure-transitional
+             (lambda _
+               ;; Keep using the old name.
+               (substitute* "setup.py"
+                 (("TRANSITIONAL = False")
+                   "TRANSITIONAL = 1"))
+               #t))))))))
+
 (define-public python2-msgpack
   (package-with-python2 python-msgpack))