tests: do not leak file descriptors

tests/test_schema2.py

@@ -12,13 +12,13 @@ def pytest_generate_tests(metafunc):
     if hasattr(metafunc, 'function'):
         fcode = metafunc.function.__code__
         if 'visualization' in fcode.co_varnames[:fcode.co_argcount]:
-            tables = json.load(
-                open(
+            with open(
                     os.path.join(
                         os.path.dirname(__file__),
                         'fixtures',
                         'schema2',
-                        'tables.json')))
+                        'tables.json')) as fd:
+                tables = json.load(fd)
             metafunc.parametrize(['visualization'], [[x] for x in tables])