The smelly.py script now also checks the Python dynamic library and
extension modules, not only the Python static library. Make also the
script more verbose: explain what it does.
The GitHub Action job now builds Python with the libpython dynamic
library.
* 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"