[ci] prefer CPython in Linux and macOS test environment (#5555)

.ci/test.sh

@@ -29,7 +29,7 @@ if [[ "$TASK" == "cpp-tests" ]]; then
     exit 0
 fi
 
-conda create -q -y -n $CONDA_ENV python=$PYTHON_VERSION
+conda create -q -y -n $CONDA_ENV "python=$PYTHON_VERSION[build=*cpython]"
 source activate $CONDA_ENV
 
 cd $BUILD_DIRECTORY
@@ -125,6 +125,7 @@ else
     DEPENDENCIES="dask=2022.7.0 distributed=2022.7.0 scipy<1.9"
 fi
 
+# re-including python=version[build=*cpython] to ensure that conda doesn't fall back to pypy
 conda install -q -y -n $CONDA_ENV \
     cloudpickle \
     ${DEPENDENCIES} \
@@ -134,12 +135,10 @@ conda install -q -y -n $CONDA_ENV \
     pandas \
     psutil \
     pytest \
+    "python=$PYTHON_VERSION[build=*cpython]" \
+    python-graphviz \
     scikit-learn || exit -1
 
-# python-graphviz has to be installed separately to prevent conda from downgrading to pypy
-conda install -q -y -n $CONDA_ENV \
-    python-graphviz || exit -1
-
 if [[ $OS_NAME == "macos" ]] && [[ $COMPILER == "clang" ]]; then
     # fix "OMP: Error #15: Initializing libiomp5.dylib, but found libomp.dylib already initialized." (OpenMP library conflict due to conda's MKL)
     for LIBOMP_ALIAS in libgomp.dylib libiomp5.dylib libomp.dylib; do sudo ln -sf "$(brew --cellar libomp)"/*/lib/libomp.dylib $CONDA_PREFIX/lib/$LIBOMP_ALIAS || exit -1; done