gnu: Fix python inputs, part 2: all inputs become native-inputs.

This patch contains the changes where all [inputs] are changed to
[native-inputs].

* gnu/packages/python.scm (python-pytest, python-fixtures,
  python-testrepository, python-virtualenv): All [inputs] are changed to
  [native-inputs].
* gnu/packages/openstack.scm (python-bandit, python-debtcollector,
  python-hacking, python-tempest-lib, python-oslo.config, python-oslo.context,
  python-oslo.i18n, python-oslo.log, python-oslo.serialization,
  python-oslosphinx, python-oslotest, python-oslo.utils): Likewise.

gnu/packages/python.scm

@@ -1608,7 +1608,7 @@ code introspection, and logging.")
            (("def test_remove_dir_prefix\\(self\\):")
             "@pytest.mark.xfail\n    def test_remove_dir_prefix(self):")))))
     (build-system python-build-system)
-    (inputs
+    (native-inputs
      `(("python-py" ,python-py)
        ("python-nose" ,python-nose)
        ("python-mock" ,python-mock)))
@@ -2042,7 +2042,7 @@ and sensible default behaviors into your setuptools run.")
     (propagated-inputs
      `(("python-six" ,python-six)
        ("python-pbr-0.11" ,python-pbr-0.11)))
-    (inputs
+    (native-inputs
      `(;; Tests
        ("python-testtools" ,python-testtools)))
     (arguments
@@ -2074,7 +2074,7 @@ Python tests.")
     (propagated-inputs
      `(("python-fixtures-0.3.16" ,python-fixtures-0.3.16)
        ("python-testtools" ,python-testtools)))
-    (inputs
+    (native-inputs
      `(("python-subunit" ,python-subunit)
        ("python-mimeparse" ,python-mimeparse)))
     (home-page "https://launchpad.net/testrepository")
@@ -2579,7 +2579,7 @@ object.")
              (substitute* "tests/test_virtualenv.py"
                (("skipif.*") "skipif(True, reason=\"Guix\")\n"))
              (zero? (system* "py.test")))))))
-    (inputs
+    (native-inputs
      `(("python-mock" ,python-mock)
        ("python-pytest" ,python-pytest)))
     (home-page "https://virtualenv.pypa.io/")