Add pytest option --warn-numpy-data-promotion

This adds a new option to make NumPy warn about data promotion behavior that has changed in NumPy 2. This new promotion can lead to lower precision results when working with floating-point scalars, and errors or overflows when working with integer scalars. Invoking pytest with `--warn-numpy-data-promotion` will cause warnings warnings to be emitted when dissimilar data types are used in an operation in such a way that NumPy ends up changing the data type of the result value.

Although this new option for Cirq's pytest code is most useful during Cirq's migration to NumPy 2, the flag will likely remain for some time afterwards too, because developers will undoubtely need time to adjust to the new NumPy behavior.

For more information about the NumPy warning enabled by this option, see
<https://numpy.org/doc/2.0/numpy_2_0_migration_guide.html#changes-to-numpy-data-type-promotion>.

conftest.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import pytest
+import numpy as np
 
 
 def pytest_addoption(parser):
@@ -25,6 +26,12 @@ def pytest_addoption(parser):
     parser.addoption(
         "--enable-slow-tests", action="store_true", default=False, help="run slow tests"
     )
+    parser.addoption(
+        "--warn-numpy-data-promotion",
+        action="store_true",
+        default=False,
+        help="enable NumPy 2 data type promotion warnings"
+    )
 
 
 def pytest_collection_modifyitems(config, items):
@@ -48,6 +55,11 @@ def pytest_collection_modifyitems(config, items):
         del skip_marks["slow"]  # pragma: no cover
     skip_keywords = frozenset(skip_marks.keys())
 
+    # If requested, globally enable verbose NumPy 2 warnings about data type
+    # promotion. See https://numpy.org/doc/2.0/numpy_2_0_migration_guide.html.
+    if config.option.warn_numpy_data_promotion:
+        np._set_promotion_state("weak_and_warn")
+
     for item in items:
         for k in skip_keywords.intersection(item.keywords):
             item.add_marker(skip_marks[k])