The link has this recommendation:
If you encounter this warning, it is strongly recommended you upgrade to a
newer Python version, or that you use pyOpenSSL as described in the
OpenSSL / PyOpenSSL section.
On Debian/Ubuntu, the python version can be determined like this:
PYTHONVERSION=$(dpkg-query --show --showformat '${Version}\n' python)
This yields "2.7.5-5ubuntu3" at the moment.
Then, compare it to the minimum version 2.7.9 to stop this error from occuring:
dpkg --compare-versions $PYTHONVERSION ge 2.7.9
If it fails (error code $? = 1), it means that python is too old and
you need to install the OpenSSL stuff:
In the file letsencrypt-auto, insert
the line
$VENV_BIN/pip install -U pyopenssl ndg-httpsclient pyasn1
after the line
$VENV_BIN/pip install -U letsencrypt letsencrypt-apache
and the lines
printf .
$VENV_BIN/pip install -U pyopenssl ndg-httpsclient pyasn1
after the line
$VENV_BIN/pip install -U letsencrypt > /dev/null
That seems to do the trick.