tests: keep numpy being optional

Ref: https://github.com/pytest-dev/pytest/pull/5950#discussion_r335254774

setup.py

@@ -28,7 +28,6 @@ def main():
                 "mock",
                 "nose",
                 "requests",
-                "numpy",
                 "xmlschema",
             ]
         },

testing/python/fixtures.py

@@ -4190,6 +4190,8 @@ def test_indirect_fixture_does_not_break_scope(testdir):
 
 
 def test_fixture_parametrization_nparray(testdir):
+    pytest.importorskip("numpy")
+
     testdir.makepyfile(
         """
         from numpy import linspace