make-release: use a fresh virtualenv for building the release

A user-provided virtualenv could contain extra packages or different
versions, which may mask problems inside Kallithea.

scripts/make-release

@@ -2,6 +2,22 b''
 set -e
 set -x
 
+cleanup()
+{
+  echo "Removing venv $venv"
+  rm  -rf "$venv"
+}
+
+echo "Checking that you are NOT inside a virtualenv"
+[ -z "$VIRTUAL_ENV" ]
+
+venv=$(mktemp -d --tmpdir kallithea-release-XXXXX)
+trap cleanup EXIT
+
+echo "Setting up a fresh virtualenv in $venv"
+virtualenv -p python2 "$venv"
+. "$venv/bin/activate"
+
 echo "Install/verify tools needed for building and uploading stuff"
 pip install --upgrade -e .
 pip install --upgrade Sphinx Sphinx-PyPI-upload