gnu: borg: Run the tests with python-pytest-2.9.2.

* gnu/packages/backup.scm (borg)[native-inputs]: Use python-pytest-2.9.2.

gnu/packages/backup.scm

@@ -470,7 +470,9 @@ (define-public borg
     (native-inputs
      `(("python-cython" ,python-cython)
        ("python-setuptools-scm" ,python-setuptools-scm)
-       ("python-pytest" ,python-pytest)
+       ;; Borg 1.0.8's test suite uses 'tmpdir_factory', which was introduced in
+       ;; pytest 2.8.
+       ("python-pytest" ,python-pytest-2.9.2)
        ;; For generating the documentation.
        ("python-sphinx" ,python-sphinx)
        ("python-sphinx-rtd-theme" ,python-sphinx-rtd-theme)))