gnu: tensorflow: Provide older setuptools version.

* gnu/packages/python-xyz.scm (python-setuptools-for-tensorflow): New public
variable.
* gnu/packages/machine-learning.scm (tensorflow)[native-inputs]: Add it.

gnu/packages/machine-learning.scm

@@ -1604,6 +1604,9 @@ (define-public tensorflow
        ("protobuf:native" ,protobuf-next) ; protoc
        ("protobuf:src" ,(package-source protobuf-next))
        ("eigen:src" ,(package-source eigen-for-tensorflow))
+       ;; install_pip_packages.sh wants setuptools 39.1.0 specifically.
+       ("python-setuptools" ,python-setuptools-for-tensorflow)
+
        ;; The commit hashes and URLs for third-party source code are taken
        ;; from "tensorflow/workspace.bzl".
        ("boringssl-src"

gnu/packages/python-xyz.scm

@@ -713,6 +713,19 @@ (define-public python-setuptools
 (define-public python2-setuptools
   (package-with-python2 python-setuptools))
 
+;; The setuptools provided by Python 3.7.4 is too new for Tensorflow.
+(define-public python-setuptools-for-tensorflow
+  (hidden-package
+   (package
+     (inherit python-setuptools)
+     (version "39.1.0")
+     (source (origin
+               (inherit (package-source python-setuptools))
+               (uri (pypi-uri "setuptools" version ".zip"))
+               (sha256
+                (base32
+                 "1mzdhvfhnv4lggxa8rjl0dzqxvfx377gg5sqs57v89wrp09lwj65")))))))
+
 (define-public python-uniseg
   (package
     (name "python-uniseg")