Spack CI: install mypy with pip rather than through spack

mypy checks the sources; it is not a dependency of the build. Adding
py-mypy to the environment puts it on the concretized DAG and in the
build cache, and pins whatever version the packages ref carries. Install
it with pip in the step instead.

'--user', because the environment's view is spack-managed, and invoke it
as a module so the script directory does not need to be on PATH.

The step then echoes 'mypy --version'. This step is expected to report
errors, so without that line a failed install and a working mypy finding
problems would look the same in the log.

Not verified locally: reproducing it needs a spack environment in the
container, and the ones from the cache warm are gone. If pip refuses to
install into that python, the version line will be missing and the cause
will be obvious from the log.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: garth-wells

.github/workflows/ci.yml

@@ -155,7 +155,7 @@ jobs:
             ${{ matrix.petsc_dep }} ^adios2~libcatalyst~fortran~sz
           spack -e ci add py-fenics-dolfinx@main${{ matrix.petsc && '+petsc4py+slepc4py' || '~petsc4py~slepc4py' }} commit=${{ github.sha }} build_type=Developer
           spack -e ci add py-gmsh ^gmsh~med~fltk+external ^opencascade~draw py-pyamg py-matplotlib \
-            py-mypy py-networkx \
+            py-networkx \
             py-numba  ^llvm~clang~lldb~lld~offload~libomptarget~polly targets=x86 \
             py-pytest py-pytest-xdist py-scipy \
             "py-pyvista ^[virtuals=gl] osmesa"
@@ -232,11 +232,21 @@ jobs:
         shell: spack-bash {0}
         run: |
           spack env activate ci
+          # Installed with pip rather than added to the environment: mypy
+          # is a check on the sources, not a dependency of the build, and
+          # this keeps it off the concretized DAG and the build cache.
+          # '--user' because the environment's view is spack-managed;
+          # invoke it as a module so PATH does not matter.
+          pip install --user mypy
+          # Echo the version, so an install failure is distinguishable
+          # from mypy running and reporting errors -- this step is
+          # expected to report errors.
+          python -m mypy --version
           DISABLE_FLAGS="--disable-error-code import-not-found --disable-error-code no-any-unimported"
           cd dolfinx-src/python
-          mypy --config-file pyproject.toml -p dolfinx $DISABLE_FLAGS
-          mypy demo $DISABLE_FLAGS
-          mypy test $DISABLE_FLAGS
+          python -m mypy --config-file pyproject.toml -p dolfinx $DISABLE_FLAGS
+          python -m mypy demo $DISABLE_FLAGS
+          python -m mypy test $DISABLE_FLAGS
 
       - name: Set default DOLFINx JIT options
         run: |