Modified code to write release coverage json data to gzip file under both Py2 and Py3.

 - Legacy-Id: 16475

Author: levkowetz

ietf/utils/test_runner.py

@@ -620,8 +620,9 @@ def teardown_test_environment(self, **kwargs):
                 self.coverage_master["version"] = self.save_version_coverage
                 self.coverage_master[self.save_version_coverage] = self.coverage_data
                 if self.coverage_file.endswith('.gz'):
-                    with gzip.open(self.coverage_file, "wt", encoding='ascii') as file:
-                        json.dump(self.coverage_master, file, sort_keys=True)
+                    with gzip.open(self.coverage_file, "w") as file:
+                        json_coverage = json.dumps(self.coverage_master, sort_keys=True)
+                        file.write(json_coverage.encode())
                 else:
                     with open(self.coverage_file, "w") as file:
                         json.dump(self.coverage_master, file, indent=2, sort_keys=True)