[CI] Bump Python 2 builder to Ubuntu 18.04

This change bumps the builder that uses the system Python 2 to run SCons
and the system Python 3 for the Python interface to Ubuntu 18.04 from
16.04. The reason for this change is that Ubuntu 16.04 provides NumPy
1.11, which does not support flexible dtypes when creating arrays
using .full(). We have decided to drop support for NumPy older than 1.12
for this reason. Numpy 1.12 was released in January 2017.

.github/workflows/main.yml

@@ -151,10 +151,10 @@ jobs:
           --exclude='/doxygen/xml' --delete --delete-excluded \
           "${DOCS_OUTPUT_DIR}" ${RSYNC_USER}@${RSYNC_SERVER}:${RSYNC_DEST}
 
-  ubuntu-1604-py2:
-    # Ubuntu 16.04 using Python 2 to run SCons and the system Python 3 for the interface
-    name: Python 2 running SCons on Ubuntu 16.04
-    runs-on: ubuntu-16.04
+  ubuntu-1804-py2:
+    # Ubuntu 18.04 using Python 2 to run SCons and the system Python 3 for the interface
+    name: Python 2 running SCons on Ubuntu 18.04
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
         name: Checkout the repository
@@ -163,11 +163,9 @@ jobs:
       - name: Install Apt dependencies
         run: |
           sudo apt-get install libboost-dev gfortran scons python3-numpy \
-          python3-pip python3-setuptools libsundials-serial-dev liblapack-dev \
+          python3-pip python3-setuptools python3-h5py python3-pandas \
+          python3-ruamel.yaml cython libsundials-serial-dev liblapack-dev \
           libblas-dev
-      - name: Install Python dependencies
-        run: |
-          sudo -H /usr/bin/python3 -m pip install ruamel.yaml cython h5py
       - name: Build Cantera
         run: scons build python_cmd=/usr/bin/python3 blas_lapack_libs=lapack,blas -j2
       - name: Test Cantera