contrib/freeze_packages: should not use too new python interpreter

Certain dependencies are only needed on old python versions,
e.g. backports of stdlib functionality.
We should definitely not use newer python when running freeze_packages.sh
than what we bundle in the binaries. Perhaps it is prudent to use the
min python version that we support (which is atm older than what we bundle).

contrib/freeze_packages.sh

@@ -6,13 +6,23 @@ set -e
 venv_dir=~/.electrum-venv
 contrib=$(dirname "$0")
 
+# note: we should not use a higher version of python than what the binaries bundle
+if [[ ! "$SYSTEM_PYTHON" ]] ; then
+    SYSTEM_PYTHON=$(which python3.6) || printf ""
+else
+    SYSTEM_PYTHON=$(which $SYSTEM_PYTHON) || printf ""
+fi
+if [[ ! "$SYSTEM_PYTHON" ]] ; then
+    echo "Please specify which python to use in \$SYSTEM_PYTHON" && exit 1;
+fi
+
 which virtualenv > /dev/null 2>&1 || { echo "Please install virtualenv" && exit 1; }
-python3 -m hashin -h > /dev/null 2>&1 || { python3 -m pip install hashin; }
-other_python=$(which python3)
+
+${SYSTEM_PYTHON} -m hashin -h > /dev/null 2>&1 || { ${SYSTEM_PYTHON} -m pip install hashin; }
 
 for i in '' '-hw' '-binaries' '-wine-build' '-mac-build'; do
     rm -rf "$venv_dir"
-    virtualenv -p $(which python3) $venv_dir
+    virtualenv -p ${SYSTEM_PYTHON} $venv_dir
 
     source $venv_dir/bin/activate
 
@@ -23,7 +33,7 @@ for i in '' '-hw' '-binaries' '-wine-build' '-mac-build'; do
     echo "OK."
 
     requirements=$(pip freeze --all)
-    restricted=$(echo $requirements | $other_python $contrib/deterministic-build/find_restricted_dependencies.py)
+    restricted=$(echo $requirements | ${SYSTEM_PYTHON} $contrib/deterministic-build/find_restricted_dependencies.py)
     requirements="$requirements $restricted"
 
     echo "Generating package hashes..."
@@ -32,7 +42,7 @@ for i in '' '-hw' '-binaries' '-wine-build' '-mac-build'; do
 
     for requirement in $requirements; do
         echo -e "\r  Hashing $requirement..."
-        $other_python -m hashin -r $contrib/deterministic-build/requirements${i}.txt ${requirement}
+        ${SYSTEM_PYTHON} -m hashin -r $contrib/deterministic-build/requirements${i}.txt ${requirement}
     done
 
     echo "OK."