Typing annotations for arithmetic overrides (e.g., DataArray + Dataset) (#9688)

* Typing annotations for arithmetic overrides (e.g., DataArray + Dataset)

This change explicitly adds annotations for arithmetic like
DataArray + Dataset in `_typed_ops.py`.

Note: I've added a dependency on jinja2, a standard Python templating
engine. It is only required for manually regenerating typed operations,
which should impact only a handful of xarray developers.

Fixes GH9656

* Update mypy on GitHub CI to use python 3.12

* add jinja2 to dev dependencies

Author: shoyer

.github/workflows/ci-additional.yaml

@@ -92,7 +92,7 @@ jobs:
         shell: bash -l {0}
     env:
       CONDA_ENV_FILE: ci/requirements/environment.yml
-      PYTHON_VERSION: "3.11"
+      PYTHON_VERSION: "3.12"
 
     steps:
       - uses: actions/checkout@v4

pyproject.toml

@@ -37,6 +37,7 @@ accel = ["scipy", "bottleneck", "numbagg", "numba>=0.54", "flox", "opt_einsum"]
 complete = ["xarray[accel,etc,io,parallel,viz]"]
 dev = [
   "hypothesis",
+  "jinja2",
   "mypy",
   "pre-commit",
   "pytest",