Use python3 in shebang

[frakman1]

https://github.com/coffeeandscripts/sqlcrush/blob/502a583e97a84efdeb48e59f1bfe403daa9681ee/bin/sqlcrush#L1

The instructions call for using python3 to install manually, yet the tool's shebang looks for python. This causes the expected error below because the dependency was installed in python3 and not python:

ImportError: No module named sqlalchemy

The shebang should say:

#!/usr/bin/env python3

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Installation notes (Ubuntu):

sudo apt install postgresql
sudo apt-get install libpq-dev