pin package versions for python-pip to bionic-security
pin package versions for python-pip to bionic-security

so that we do not get an update of libssl to 1.1.1, since the stack is still on 1.1.0

ad-m/heroku-buildpack-python,skilljar/heroku-buildpack-python,Scalingo/python-buildpack,AltSchool/heroku-buildpack-python,heroku/heroku-buildpack-python,heroku/heroku-buildpack-python,benmurden/heroku-buildpack-python-ember,Scalingo/python-buildpack,benmurden/heroku-buildpack-python-ember,ad-m/heroku-buildpack-python,AltSchool/heroku-buildpack-python