Fix format issues flagged by black

When I run `check/format-incremental`, it reports two trivial
formatting errors as shown below. This commit fixes that.

```
Comparing against revision 'upstream/main'.
Running the black formatter... (version: black, 25.1.0 (compiled: yes)
Python (CPython) 3.10.14)
--- cirq-core/cirq/ops/op_tree.py	2025-01-30 20:52:25.721992+00:00
+++ cirq-core/cirq/ops/op_tree.py	2025-02-23 18:32:17.923984+00:00
@@ -10,12 +10,11 @@
 # distributed under the License is distributed on an "AS IS" BASIS,
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.

-"""A recursive type describing trees of operations, and utility methods for it.
-"""
+"""A recursive type describing trees of operations, and utility methods for it."""

 from typing import Callable, Iterable, Iterator, NoReturn, Union, TYPE_CHECKING

 from cirq._doc import document
 from cirq._import import LazyLoader
would reformat cirq-core/cirq/ops/op_tree.py
--- cirq-core/cirq/transformers/dynamical_decoupling.py	2025-01-30 20:52:25.732467+00:00
+++ cirq-core/cirq/transformers/dynamical_decoupling.py	2025-02-23 18:32:18.291548+00:00
@@ -81,11 +81,11 @@
             f' identity up to a global phase, got {product}.'.replace('\n', ' ')
         )

 def _parse_dd_sequence(
-    schema: Union[str, Tuple[ops.Gate, ...]]
+    schema: Union[str, Tuple[ops.Gate, ...]],
 ) -> Tuple[Tuple[ops.Gate, ...], Dict[ops.Gate, ops.Pauli]]:
     """Parses and returns dynamical decoupling sequence and its associated pauli map from schema."""
     dd_sequence = None
     if isinstance(schema, str):
         dd_sequence = _get_dd_sequence_from_schema_name(schema)
would reformat cirq-core/cirq/transformers/dynamical_decoupling.py

Oh no! 💥 💔 💥
2 files would be reformatted, 167 files would be left unchanged.
```

cirq-core/cirq/ops/op_tree.py

@@ -12,8 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""A recursive type describing trees of operations, and utility methods for it.
-"""
+"""A recursive type describing trees of operations, and utility methods for it."""
 
 from typing import Callable, Iterable, Iterator, NoReturn, Union, TYPE_CHECKING
 

cirq-core/cirq/transformers/dynamical_decoupling.py

@@ -83,7 +83,7 @@ def _validate_dd_sequence(dd_sequence: Tuple[ops.Gate, ...]) -> None:
 
 
 def _parse_dd_sequence(
-    schema: Union[str, Tuple[ops.Gate, ...]]
+    schema: Union[str, Tuple[ops.Gate, ...]],
 ) -> Tuple[Tuple[ops.Gate, ...], Dict[ops.Gate, ops.Pauli]]:
     """Parses and returns dynamical decoupling sequence and its associated pauli map from schema."""
     dd_sequence = None