Upgrade pex to newer version and remove wheel from build.txt
Upgrade pex to newer version and remove wheel from build.txt

indirectlylit/kolibri,learningequality/kolibri,learningequality/kolibri,mrpau/kolibri,mrpau/kolibri,learningequality/kolibri,mrpau/kolibri,learningequality/kolibri,indirectlylit/kolibri,mrpau/kolibri,indirectlylit/kolibri,indirectlylit/kolibri