Remove unnecessary copy operations

There is no need to copy "case_dict" inside serialization test
functions in test_metadata_serialization.py when we are testing
invalid arguments.
These dictionaries are not be used after calling "from_dict" and
it doesn't matter if they are empty afterward.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tests/test_metadata_serialization.py

@@ -137,7 +137,7 @@ def test_invalid_signatures_serialization(self, test_data: str) -> None:
     def test_invalid_signed_serialization(self, test_case_data: str) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises((KeyError, ValueError, TypeError)):
-            Snapshot.from_dict(copy.deepcopy(case_dict))
+            Snapshot.from_dict(case_dict)
 
     valid_keys: utils.DataSet = {
         "all": '{"keytype": "rsa", "scheme": "rsassa-pss-sha256", \
@@ -170,7 +170,7 @@ def test_invalid_key_serialization(self, test_case_data: str) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises((TypeError, KeyError)):
             keyid = case_dict.pop("keyid")
-            Key.from_dict(keyid, copy.copy(case_dict))
+            Key.from_dict(keyid, case_dict)
 
     invalid_roles: utils.DataSet = {
         "no threshold": '{"keyids": ["keyid"]}',
@@ -184,7 +184,7 @@ def test_invalid_key_serialization(self, test_case_data: str) -> None:
     def test_invalid_role_serialization(self, test_case_data: str) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises((KeyError, TypeError, ValueError)):
-            Role.from_dict(copy.deepcopy(case_dict))
+            Role.from_dict(case_dict)
 
     valid_roles: utils.DataSet = {
         "all": '{"keyids": ["keyid"], "threshold": 3}',
@@ -291,7 +291,7 @@ def test_root_serialization(self, test_case_data: str) -> None:
     def test_invalid_root_serialization(self, test_case_data: str) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises(ValueError):
-            Root.from_dict(copy.deepcopy(case_dict))
+            Root.from_dict(case_dict)
 
     invalid_metafiles: utils.DataSet = {
         "wrong length type": '{"version": 1, "length": "a", "hashes": {"sha256" : "abc"}}',
@@ -307,7 +307,7 @@ def test_invalid_root_serialization(self, test_case_data: str) -> None:
     def test_invalid_metafile_serialization(self, test_case_data: str) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises((TypeError, ValueError, AttributeError)):
-            MetaFile.from_dict(copy.deepcopy(case_dict))
+            MetaFile.from_dict(case_dict)
 
     valid_metafiles: utils.DataSet = {
         "all": '{"hashes": {"sha256" : "abc"}, "length": 12, "version": 1}',
@@ -331,7 +331,7 @@ def test_metafile_serialization(self, test_case_data: str) -> None:
     def test_invalid_timestamp_serialization(self, test_case_data: str) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises((ValueError, KeyError)):
-            Timestamp.from_dict(copy.deepcopy(case_dict))
+            Timestamp.from_dict(case_dict)
 
     valid_timestamps: utils.DataSet = {
         "all": '{ "_type": "timestamp", "spec_version": "1.0.0", "version": 1, "expires": "2030-01-01T00:00:00Z", \
@@ -407,7 +407,7 @@ def test_invalid_delegated_role_serialization(
     ) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises(ValueError):
-            DelegatedRole.from_dict(copy.copy(case_dict))
+            DelegatedRole.from_dict(case_dict)
 
     invalid_delegations: utils.DataSet = {
         "empty delegations": "{}",
@@ -468,7 +468,7 @@ def test_invalid_delegation_serialization(
     ) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises((ValueError, KeyError, AttributeError)):
-            Delegations.from_dict(copy.deepcopy(case_dict))
+            Delegations.from_dict(case_dict)
 
     valid_delegations: utils.DataSet = {
         "all": '{"keys": { \
@@ -505,7 +505,7 @@ def test_invalid_targetfile_serialization(
     ) -> None:
         case_dict = json.loads(test_case_data)
         with self.assertRaises(KeyError):
-            TargetFile.from_dict(copy.deepcopy(case_dict), "file1.txt")
+            TargetFile.from_dict(case_dict, "file1.txt")
 
     valid_targetfiles: utils.DataSet = {
         "all": '{"length": 12, "hashes": {"sha256" : "abc"}, \