fix: numpy 1.20 has dropped Python 3.6

Author: oesteban

.github/workflows/pythonpackage.yml

@@ -142,6 +142,9 @@ jobs:
         source /tmp/setup_install/bin/activate
         python -m pip install -U "setuptools ~= 42.0" wheel "setuptools_scm[toml] >= 3.4" \
                               setuptools_scm_git_archive "${{ matrix.pip }}"
+        if [ "${{ matrix.python-version }}" == "3.6" ]; then
+          pip install "numpy < 1.20"
+        fi
         python setup.py install
         INSTALLED_VERSION=$(python -c 'import templateflow as tf; print(tf.__version__, end="")')
         echo "INSTALLED: \"${INSTALLED_VERSION}\""
@@ -174,6 +177,9 @@ jobs:
         source /tmp/setup_develop/bin/activate
         python -m pip install -U "setuptools ~= 42.0" wheel "setuptools_scm[toml] >= 3.4" \
                               setuptools_scm_git_archive "${{ matrix.pip }}"
+        if [ "${{ matrix.python-version }}" == "3.6" ]; then
+          pip install "numpy < 1.20"
+        fi
         python setup.py develop
         INSTALLED_VERSION=$(python -c 'import templateflow as tf; print(tf.__version__, end="")')
         echo "INSTALLED: \"${INSTALLED_VERSION}\""