[CI] Install typing_extensions on CI

Python 3.7 requires the typing_extensions module. Scope it to 3.7 only
for now, so it's more obvious when it can be removed.

.github/workflows/main.yml

@@ -47,6 +47,9 @@ jobs:
     - name: Install Python dependencies
       run: python3 -m pip install ruamel.yaml scons==3.1.2 numpy cython h5py pandas
         pytest pytest-github-actions-annotate-failures
+    - name: Install typing_extensions for Python 3.7
+      if: matrix.python-version == '3.7'
+      run: python3 -m pip install typing_extensions
     - name: Build Cantera
       run: python3 `which scons` build env_vars=all -j2 debug=n --debug=time
     - name: Test Cantera
@@ -118,6 +121,9 @@ jobs:
     - name: Install Python dependencies
       run: python3 -m pip install ruamel.yaml scons numpy cython h5py pandas pytest
         pytest-github-actions-annotate-failures
+    - name: Install typing_extensions for Python 3.7
+      if: matrix.python-version == '3.7'
+      run: python3 -m pip install typing_extensions
     - name: Build Cantera
       run: python3 `which scons` build env_vars=all -j3 debug=n --debug=time
     - name: Test Cantera
@@ -287,6 +293,9 @@ jobs:
         run: python3 -m pip install -U pip setuptools wheel
       - name: Install Python dependencies
         run: python3 -m pip install ruamel.yaml scons numpy cython h5py pandas matplotlib scipy
+      - name: Install typing_extensions for Python 3.7
+        if: matrix.python-version == '3.7'
+        run: python3 -m pip install typing_extensions
       - name: Build Cantera
         run: python3 `which scons` build -j2 debug=n
       - name: Run the examples
@@ -427,6 +436,9 @@ jobs:
       run: |
         mamba install -q scons numpy cython ruamel.yaml boost-cpp eigen yaml-cpp h5py pandas pytest
       shell: pwsh
+    - name: Install typing_extensions for Python 3.7
+      if: matrix.python-version == '3.7'
+      run: mamba install -q typing_extensions
     - name: Build Cantera
       run: scons build system_eigen=y system_yamlcpp=y VERBOSE=True
         msvc_version=${{ matrix.vs-toolset }} f90_interface=n debug=n --debug=time -j2
@@ -479,6 +491,9 @@ jobs:
         run: |
           python -m pip install -U pip setuptools wheel
           python -m pip install scons pypiwin32 numpy ruamel.yaml cython h5py pandas pytest pytest-github-actions-annotate-failures
+      - name: Install typing_extensions for Python 3.7
+        if: matrix.python-version == '3.7'
+        run: python -m pip install typing_extensions
       - name: Restore Boost cache
         uses: actions/cache@v2
         id: cache-boost