ci: install python3.7 using pyenv

This is sub-optimal, ideally all python version should be installed
using the same approach.

install.sh

@@ -23,10 +23,16 @@ function install_python_ppa {
 
 function install_python {
   local py=$1
-  apt-get install -y $py $py-dev
-  curl --silent --show-error --retry 5 https://bootstrap.pypa.io/get-pip.py | $py
-  $py -m pip install --upgrade pip
-  $py -m pip install --upgrade virtualenv
+  if [[ "$py" != "python3.7" ]]; then
+    apt-get install -y $py $py-dev
+    curl --silent --show-error --retry 5 https://bootstrap.pypa.io/get-pip.py | $py
+    $py -m pip install --upgrade pip
+    $py -m pip install --upgrade virtualenv
+  else
+    # See https://discuss.circleci.com/t/install-python-3-7-in-machine-executor/25883/5
+    pyenv versions
+    pyenv global 3.7.1
+  fi
 }
 
 function init_venv {