To use pip, you must be running Jython v2.7.1 (downloaded from here: http://search.maven.org/remotecontent?filepath=org/python/jython-standalone/2.7.1/jython-standalone-2.7.1.jar). This is not compatible with Esper, so be sure to remove all of these files before upgrading, ie:
rm -rf ./automation/jsr223/000_components/000_Esper.py ./automation/lib/python/esper
With v2.7.1 installed, we can run the Jython from the openHAB installation to install pip
java -jar /openhab/conf/automation/jython/jython-standalone-2.7.1.jar -m ensurepip