"Problem: both package blockdiag-1.5.4-4.fc30.noarch and fedora-obsolete-packages-30-40.noarch obsolete python2-blockdiag < 1.5.4-2" seems like this will get fixed when this is moved form updates testing. I hope.
Updating from selinux-policy-targeted-3.14.2-49.fc29 to selinux-policy-targeted-3.14.2-51.fc29 - the scriptlet turned my CPU into a grill, my X session and Xfce desktop crashed and dnf failed mid transaction.
Let me just say that if other RPM packages actually depend on /usr/bin/python3, you'll get /usr/bin/python3. However if non RPM software you use depends on it, you are now in trouble. So what you expect is a reasonable thing to ask.