Fix Mock 4.0.3 compatibility - Unpatch only once during teardown (#3541)

* #3535 - Unpatch only once during teardown

* EnvVar patching - Fix Python2 bug

* Allow latest version of mock-library

tests/test_core/test_environ_patching.py

@@ -0,0 +1,34 @@
+import os
+import sure  # noqa
+from moto import mock_ec2, mock_s3
+
+KEY = "AWS_ACCESS_KEY_ID"
+
+
+def test_aws_keys_are_patched():
+    with mock_ec2():
+        patched_value = os.environ[KEY]
+        patched_value.should.equal("foobar_key")
+
+
+def test_aws_keys_can_be_none():
+    """
+    Verify that the os.environ[KEY] can be None
+    Patching the None-value shouldn't be an issue
+    """
+    original = os.environ.get(KEY, "value-set-by-user")
+    # Delete the original value by the user
+    try:
+        del os.environ[KEY]
+    except KeyError:
+        pass  # Value might not be set on this system in the first place
+    try:
+        # Verify that the os.environ[KEY] is patched
+        with mock_s3():
+            patched_value = os.environ[KEY]
+            patched_value.should.equal("foobar_key")
+        # Verify that the os.environ[KEY] is unpatched, and reverts to None
+        assert os.environ.get(KEY) is None
+    finally:
+        # Reset the value original - don't want to change the users system
+        os.environ[KEY] = original

tests/test_s3/test_s3_classdecorator.py

@@ -0,0 +1,18 @@
+import unittest
+
+import boto3
+from moto import mock_s3
+
+
+@mock_s3
+class ClassDecoratorTest(unittest.TestCase):
+    """
+    https://github.com/spulec/moto/issues/3535
+    An update to the mock-package introduced a failure during teardown.
+    This test is in place to catch any similar failures with our mocking approach
+    """
+
+    def test_instantiation_succeeds(self):
+        s3 = boto3.client("s3", region_name="us-east-1")
+
+        assert s3 is not None