Bump mypy from 1.14.1 to 1.15.0 in /dev_tools/requirements (#7037)

* Bump mypy from 1.14.1 to 1.15.0 in /dev_tools/requirements

Bumps [mypy](https://github.com/python/mypy) from 1.14.1 to 1.15.0.
- [Changelog](https://github.com/python/mypy/blob/master/CHANGELOG.md)
- [Commits](python/mypy@v1.14.1...v1.15.0)

---
updated-dependencies:
- dependency-name: mypy
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

* Address type annotations issues from mypy-1.15.0

Use consistent type for initial values of local variables.

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Pavol Juhas <juhas@google.com>

cirq-google/cirq_google/line/placement/anneal.py

@@ -248,7 +248,7 @@ def extract_sequences() -> List[List[cirq.GridQubit]]:
             """
             seqs = []
             prev = None
-            seq = None
+            seq = []
             for node in self._c:
                 if prev is None:
                     seq = [node]

cirq-google/cirq_google/line/placement/greedy.py

@@ -230,7 +230,7 @@ class _PickLargestArea(GreedySequenceSearch):
     def _choose_next_qubit(self, qubit: GridQubit, used: Set[GridQubit]) -> Optional[GridQubit]:
         analyzed: Set[GridQubit] = set()
         best = None
-        best_size = None
+        best_size = 0
         for m in self._c_adj[qubit]:
             if m not in used and m not in analyzed:
                 reachable = self._collect_unused(m, used)

dev_tools/requirements/deps/mypy.txt

@@ -1,5 +1,5 @@
 # the mypy dependency file
-mypy==1.14.1
+mypy==1.15.0
 
 # packages with stub types for various libraries
 types-protobuf>=4.24