gnu: python-toolz: Enable tests.

* gnu/packages/python-xyz.scm (python-toolz)[arguments]: Remove #:tests?.
Override check phase.
[native-inputs]: Add python-pytest.

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

gnu/packages/python-xyz.scm

@@ -21296,9 +21296,17 @@ library's @code{threading} module.")
         (base32
          "1j9i7fdjnx9dz35fdj5gvgxx6585ja9sxgaiv65if77nlxz0m7wj"))))
     (build-system python-build-system)
-    ;; FIXME: tests cannot be computed: "Can't perform this operation for
-    ;; unregistered loader type"
-    (arguments '(#:tests? #f))
+    (arguments
+     `(#:phases
+       (modify-phases %standard-phases
+         (replace 'check
+           (lambda* (#:key inputs outputs tests? #:allow-other-keys)
+             (when tests?
+               (add-installed-pythonpath inputs outputs)
+               (invoke "python" "-m" "pytest" "--doctest-modules"
+                       "--pyargs" "toolz")))))))
+    (native-inputs
+     `(("python-pytest" ,python-pytest)))
     (home-page "https://github.com/pytoolz/toolz/")
     (synopsis "List processing tools and functional utilities")
     (description