[CI][Python] Use Python 3 on macOS

Currently, Python 2 is used.

.github/workflows/python.yml

@@ -160,6 +160,7 @@ jobs:
       ARROW_WITH_SNAPPY: ON
       ARROW_WITH_BROTLI: ON
       ARROW_BUILD_TESTS: OFF
+      CMAKE_ARGS: "-DPYTHON_EXECUTABLE=/usr/local/bin/python3"
     steps:
       - name: Checkout Arrow
         uses: actions/checkout@v1
@@ -170,12 +171,13 @@ jobs:
         run: |
           brew bundle --file=cpp/Brewfile
           brew install coreutils python
-          pip install -r python/requirements.txt \
-                      -r python/requirements-test.txt \
-                      cython
+          pip3 install -r python/requirements.txt \
+                       -r python/requirements-test.txt \
+                       cython
       - name: Build
         shell: bash
         run: |
+          export PYTHON=python3
           ci/scripts/cpp_build.sh $(pwd)
           ci/scripts/python_build.sh $(pwd)
       - name: Test

ci/scripts/python_build.sh

@@ -39,8 +39,9 @@ pushd ${source_dir}
 relative_build_dir=$(realpath --relative-to=. $build_dir)
 
 # not nice, but prevents mutating the mounted the source directory for docker
-python setup.py build --build-base $build_dir \
-                install --single-version-externally-managed \
-                        --record $relative_build_dir/record.txt
+${PYTHON:-python} \
+  setup.py build --build-base $build_dir \
+           install --single-version-externally-managed \
+                   --record $relative_build_dir/record.txt
 
 popd