I think letsencrypt client needs python 2.7 at least if @schoen can confirm
For myself, i have my own Centmin Mod LEMP web stack which can install 3rd party YUM repos for missing packages. One of those is for Rackspace backed IUS Community YUM repo https://iuscommunity.org/pages/Repos.html which supports RHEL/CentOS 5, 6, 7 and has python 2.7, 3.2, 3.3, 3.4 available as side packages which don’t interfere with existing default OS python versions.
Once IUS Community YUM repo is installed you can install Python 2.7 via yum
yum -y install python27 python27-devel python27-pip python27-setuptools python27-tools python27-virtualenv --enablerepo=ius
so you can call python 2.7 via python2.7 command
so instead of
virtualenv --no-site-packages -p python2 venv
virtualenv --no-site-packages -p python2.7 venv
FYI, IUS Community Repo requires the EPEL Repo installed too.
edit: unfortunately still ran into other problems on CentOS 6 https://github.com/letsencrypt/letsencrypt/issues/734#issuecomment-135944187 seems python 2.6 for centos is the one that works ! edit: update again python 2.7 works too now