Parenthesize dict-tuple to pacify pypy3.7 parser

In pypy3, the following line is invalid:

    x: Tuple[dict, dict] = {}, {}

but this variant *is* valid:

    x: Tuple[dict, dict] = ({}, {})

Apply this correction where it occurs in test_validators.py

Author: sirosen

jsonschema/tests/test_validators.py

@@ -1663,7 +1663,7 @@ def test_False_is_not_a_schema_even_if_you_forget_to_check(self):
 
 class TestDraft3Validator(AntiDraft6LeakMixin, ValidatorTestMixin, TestCase):
     Validator = validators.Draft3Validator
-    valid: typing.Tuple[dict, dict] = {}, {}
+    valid: typing.Tuple[dict, dict] = ({}, {})
     invalid = {"type": "integer"}, "foo"
 
     def test_any_type_is_valid_for_type_any(self):
@@ -1695,31 +1695,31 @@ def test_is_type_does_not_evade_bool_if_it_is_being_tested(self):
 
 class TestDraft4Validator(AntiDraft6LeakMixin, ValidatorTestMixin, TestCase):
     Validator = validators.Draft4Validator
-    valid: typing.Tuple[dict, dict] = {}, {}
+    valid: typing.Tuple[dict, dict] = ({}, {})
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft6Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft6Validator
-    valid: typing.Tuple[dict, dict] = {}, {}
+    valid: typing.Tuple[dict, dict] = ({}, {})
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft7Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft7Validator
-    valid: typing.Tuple[dict, dict] = {}, {}
+    valid: typing.Tuple[dict, dict] = ({}, {})
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft201909Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft201909Validator
-    valid: typing.Tuple[dict, dict] = {}, {}
+    valid: typing.Tuple[dict, dict] = ({}, {})
     invalid = {"type": "integer"}, "foo"
 
 
 class TestDraft202012Validator(ValidatorTestMixin, TestCase):
     Validator = validators.Draft202012Validator
-    valid: typing.Tuple[dict, dict] = {}, {}
+    valid: typing.Tuple[dict, dict] = ({}, {})
     invalid = {"type": "integer"}, "foo"