Fix test_show_require_invalid_version with pkg_resources backend

The pkg_ressources metadata backend does not suffer from invalid metadata in this test case.

tests/functional/test_invalid_versions_and_specifiers.py

@@ -2,6 +2,7 @@
 
 import pytest
 
+from pip._internal.metadata import select_backend
 from tests.lib import PipTestEnvironment, TestData
 
 
@@ -130,4 +131,9 @@ def test_show_require_invalid_version(
     result = script.pip("show", "require-invalid-version")
     assert "Name: require-invalid-version\nVersion: 1.0\n" in result.stdout
     assert "Requires: invalid-version ==2010i\n" in result.stdout
-    assert "Required-by: #N/A\n" in result.stdout
+    if select_backend().NAME == "importlib":
+        assert "Required-by: #N/A\n" in result.stdout
+    elif select_backend().NAME == "pkg_resources":
+        assert "Required-by: \n" in result.stdout
+    else:
+        assert False, "Unknown metadata backend"