[CI] Switch to Homebrew Python on macOS

This avoids weird linker issues where the GitHub Actions Python
required linkage to libintl but the only available version of
libintl, installed in Homebrew, was targeting a different macOS
version.

.github/workflows/main.yml

@@ -108,26 +108,22 @@ jobs:
       name: Checkout the repository
       with:
         submodules: recursive
-    - name: Setup Python
-      uses: actions/setup-python@v2
-      with:
-        python-version: ${{ matrix.python-version }}
-        architecture: x64
     - name: Install Brew dependencies
-      run: brew install boost libomp
+      run: |
+        brew install boost libomp scons python@${{ matrix.python-version }}
     - name: Upgrade pip
       run: python3 -m pip install -U pip 'setuptools>=47.0.0,<48' wheel
     - name: Install Python dependencies
-      run: python3 -m pip install ruamel.yaml scons numpy cython h5py pandas pytest
+      run: python3 -m pip install ruamel.yaml 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
+      run: scons build env_vars=all -j3 debug=n --debug=time
     - name: Test Cantera
       run:
-        python3 `which scons` test show_long_tests=yes verbose_tests=yes --debug=time
+        scons test show_long_tests=yes verbose_tests=yes --debug=time
 
   # Coverage is its own job because macOS builds of the samples
   # use Homebrew gfortran which is not compatible for coverage

site_scons/buildutils.py

@@ -1301,10 +1301,10 @@ def setup_python_env(env):
     env["site_packages"] = info["site_packages"]
     env["user_site_packages"] = info["user_site_packages"]
     if env["OS"] != "Windows":
-        env["py_libpath"] = info["LIBPL"]
+        env["py_libpath"] = [info["LIBPL"], info["LIBDIR"]]
         py_lib = "python" + info["py_version_short"] + info["abiflags"]
     else:
-        env["py_libpath"] = info["installed_base"] + "\\libs"
+        env["py_libpath"] = [info["installed_base"] + "\\libs"]
         py_lib = "python" + py_version_nodot
     env["py_libs"] = [py_lib] + [lib[2:] for lib in info.get("LIBS", "").split()
                                  if lib.startswith("-l")]

src/SConscript

@@ -97,7 +97,7 @@ if env["python_package"] == "full":
     localenv.Append(LIBS=pyenv["py_libs"], LIBPATH=pyenv["py_libpath"])
     env["cantera_libs"].extend(pyenv["py_libs"])
     env.Append(LIBPATH=pyenv["py_libpath"])
-    env["extra_lib_dirs"].append(pyenv["py_libpath"])
+    env["extra_lib_dirs"].extend(pyenv["py_libpath"])
 
 # build the Cantera static library
 lib = build(localenv.StaticLibrary('../lib/cantera', libraryTargets,