[ci] put ceiling on graphviz in Python 3.7 jobs (#6370)

.ci/test.sh

@@ -126,9 +126,9 @@ fi
 # older versions of Dask are incompatible with pandas>=2.0, but not all conda packages' metadata accurately reflects that
 #
 # ref: https://github.com/microsoft/LightGBM/issues/6030
-CONSTRAINED_DEPENDENCIES="'dask>=2023.5.0' 'distributed>=2023.5.0' 'pandas>=2.0'"
+CONSTRAINED_DEPENDENCIES="'dask>=2023.5.0' 'distributed>=2023.5.0' 'pandas>=2.0' python-graphviz"
 if [[ $PYTHON_VERSION == "3.7" ]]; then
-    CONSTRAINED_DEPENDENCIES="'dask' 'distributed' 'pandas<2.0'"
+    CONSTRAINED_DEPENDENCIES="'dask' 'distributed' 'python-graphviz<0.20.2' 'pandas<2.0'"
 fi
 
 # including python=version[build=*cpython] to ensure that conda doesn't fall back to pypy
@@ -143,7 +143,6 @@ mamba create -q -y -n $CONDA_ENV \
     pyarrow \
     pytest \
     ${CONDA_PYTHON_REQUIREMENT} \
-    python-graphviz \
     scikit-learn \
     scipy || exit 1