lspclient: use pyls entry script for python LSP server

... which ensures direct exec failure if not present rather than only indirectly when invoking interpreter

All in all, this should make it easier/clearer to start (or fail to start) the python-language server, and not make it look like it is crashing if it is simply not present. The pyls script should be present when installed the python-pip way, and presumably also if in some distro way. Suffice to say it works in my setting, but the Pull Request is here to check/verify it also works in other settings. And then to merge if indeed so.