move requirements to home directory first.

bin/steps/pip-uninstall

@@ -3,7 +3,11 @@ set +e
 bpwatch start pip_uninstall
 if [[ -f .heroku/python/requirements-declared.txt ]]; then
 
-  pip-diff --stale .heroku/python/requirements-declared.txt requirements.txt > .heroku/python/requirements-stale.txt
+  cp .heroku/python/requirements-declared.txt requirements-declared.txt
+
+  pip-diff --stale requirements-declared.txt requirements.txt > .heroku/python/requirements-stale.txt
+
+  rm -fr requirements-declared.txt
 
   if [[ -s .heroku/python/requirements-stale.txt ]]; then
     puts-step "Uninstalling stale dependencies"