Make subprocess.Popen call shell-agnostic

The shell command type -p npm is valid in bash but the -p option is not valid in sh or dash (the default shell - rather than 'login shell' - in many Linux).

However, type npm gives the same result in all three shells: "npm is /usr/bin/npm"; so taking the last word from that result should give us what we need.

https://yourlabs.io/oss/djsnowpack/-/blob/master/djsnowpack.py#L56-62

It is possible to specify /bin/bash as the required shell using the executable keyword, but the shell may not be installed in the environment. The same argument applies to using the command which.

https://docs.python.org/3/library/subprocess.html#popen-constructor