PendingReleaseNotes: revise the note on python3.4 to python3

PendingReleaseNotes: revise the note on python3.4 to python3

as RHEL/CentOS 7.7 offers python3 (python3.6), EPEL7 removed the
duplicated python3-* (python36-*) packages, see
https://lists.fedoraproject.org/archives/list/epel-devel@lists.fedoraproject.org/message/GUSLTD5AJWFEGBFK5EYFVH3LHQVHUBAP/

and EPEL7 only supports the latest RHEL7.x release, which is now 7.7

so should not mention EPEL7 in this note,

for the reason why EPEL removed these packages, see EPEL's guideline:
https://fedoraproject.org/wiki/EPEL/GuidelinesAndPolicies#Policy

Signed-off-by: Kefu Chai <kchai@redhat.com>