tests(crc32c): Add Python 3.15 to unit test versions (#18077)

3.15 was missing from the nox config, leading to [unit test
errors](https://github.com/googleapis/google-cloud-python/actions/runs/31527438043/job/93899188150?pr=17642):
`Sessions not found: unit-3.15`

This PR adds the session to nox. Note that the unit tests are skipped
down-stream, but this change causes the session to trigger

Author: daniel-sanche

packages/google-crc32c/noxfile.py

@@ -25,7 +25,7 @@
 
 # Constants
 DEFAULT_PYTHON_VERSION = "3.14"
-UNIT_TEST_PYTHON_VERSIONS = ["3.10", "3.11", "3.12", "3.13", "3.14"]
+UNIT_TEST_PYTHON_VERSIONS = ["3.10", "3.11", "3.12", "3.13", "3.14", "3.15"]
 ALL_PYTHON = list(UNIT_TEST_PYTHON_VERSIONS)
 
 FLAKE8_VERSION = "flake8==6.1.0"