Formalize 3.15 support (#7422)

Author: nateprewitt

.github/workflows/run-tests.yml

@@ -8,7 +8,6 @@ permissions:
 jobs:
   build:
     runs-on: ${{ matrix.os }}
-    continue-on-error: ${{ matrix.python-version == '3.15-dev' }}
     timeout-minutes: 10
     strategy:
       fail-fast: false

pyproject.toml

@@ -37,6 +37,7 @@ classifiers = [
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
     "Programming Language :: Python :: 3.14",
+    "Programming Language :: Python :: 3.15",
     "Programming Language :: Python :: 3 :: Only",
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",