Revert "gnu: Remove python-msgpack-transitional."

This reverts commit d5ae6739fd.

It turned out that 'poetry' requires this transitional msgpack package:

https://issues.guix.gnu.org/53228

Signed-off-by: Leo Famulari <leo@famulari.name>

gnu/packages/python-xyz.scm

@@ -10923,6 +10923,40 @@ (define-public python-msgpack
     (home-page "https://pypi.org/project/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)
+    (name "python-msgpack-transitional")
+    (version "0.5.6")
+    (source (origin
+              (method url-fetch)
+              (uri (pypi-uri "msgpack" version))
+              (sha256
+               (base32
+                "1hz2dba1nvvn52afg34liijsm7kn65cmn06dl0xbwld6bb4cis0f"))))
+    (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"))
+               ;; This old version is not compatible with Python 3.9
+               (substitute* '("test/test_buffer.py" "test/test_extension.py")
+                 ((".tostring\\(") ".tobytes("))
+               (substitute* '("test/test_buffer.py" "test/test_extension.py")
+                 ((".fromstring\\(") ".frombytes("))
+               #t))))))))
+
+(define-public python2-msgpack
+  (package-with-python2 python-msgpack))
+
 (define-public python-netaddr
   (package
     (name "python-netaddr")