Try pip3 if pip is not present

[varungandhi-src]

This error seems a little unfortunate.

scip-python index . --project-name chartify
(18:23:18) Indexing /Users/varun/Code/chartify with version 5ac3a88e54cf620389741f396cc19d60fe032822
(18:23:18) Evaluating python environment dependencies
/bin/sh: pip: command not found


Experienced Fatal Error While Indexing:
Please create an issue at github.com/sourcegraph/scip-python: Error: Command failed: pip list --format=json
/bin/sh: pip: command not found

[tjdevries]

Fixed with #79