I ran into the same problem as of update to gajim 1.0.2. To make gajim work again I use code below to patch/usr/lib64/python3.6/site-packages/nbxmpp/tls_nb.py:349

try:
    lines = f.readlines()
except (UnicodeDecodeError) as e:
    log.warning(cert_path)
    f.close()
    f = open(cert_path, encoding='latin-1')
    lines = f.readlines()

You can see warning command which outputs one other file, that is apparently latin-1 encoded too
05/26/2018 13:42:15 (W) nbxmpp.tls_nb /etc/ssl/certs/ca-certificates.crt
05/26/2018 13:42:15 (W) nbxmpp.tls_nb /etc/ssl/certs/6f2c1157.0
05/26/2018 13:42:15 (W) nbxmpp.tls_nb /etc/ssl/certs/6f2c1157.pem

Edit: /etc/ssl/certs/6f2c1157.0 is a symlink to 6f2c1157.pem

Until this is solved, you can convert the files with a wrong encoding using the below command:
sudo iconv -f iso-8859-1 -t ascii//TRANSLIT 6f2c1157.pem -o 6f2c1157.pem

should we patch nbxmpp with the provided patch?

imho it's R478 that should be patched because some other applications may also dislike such encoding, but let's ask @ikey what should be done with this.
if you should patch nbxmpp, please commandeer D3011 instead of working on the version from the repository.