bpo-31810: Add smelly.py to check exported symbols (#4057)

* Add Tools/scripts/smelly.py: script checking if all symbols
  exported by libpython start with "Py" or "_Py".
* Modify "make smelly" to run smelly.py: the command now fails with a
  non-zero exit code if libpython leaks a "smelly" symbol.
* Travis CI now runs "make smelly"