Using bash array and parameter expansion to have multiline command.

01-before.bash

sudo apt-get install -y git python python-dev \
    python-setuptools libxml2-dev libxslt-dev zlib1g-dev \
    postgresql postgresql-client python-psycopg2

02-after.bash

# array are more flexible to formatting
pkg_list=( 
           git python python-dev
           python-setuptools libxml2-dev libxslt-dev zlib1g-dev
           postgresql postgresql-client python-psycopg2
         )
# the final command is short and has better readibility
sudo apt-get install -y "${pkg_list[@]}"

# Option such as '-y' or '--yes' can also be moved to the $pkg_list array