For CentOS 7, I did a
yum search PyMySQL
and then installed the version for Python 3 that appeared.
I did get an option for python2, but since we are currently in the last days of python 2 being supported, I felt confident in ignoring that.
I was just pointing out that following the LibreNMS install docs, this was the one thing I did that corrected the final error on the validation.php web page. You have a point that it may require a different version this time next year, so an accurate way to identify the correct package is helpful.