Use %epel not %rhel for Python 3 enabling check

This is more correct because Python 3 is really available in
*EPEL* not in RHEL. This is important here because the same
spec (kept in productmd SCM) is used to do builds for RHEL in
contexts without EPEL, and Python 3 building will fail in that
scenario.

python-productmd.spec

@@ -1,5 +1,7 @@
 # Enable Python 3 builds for Fedora + EPEL >5
-%if 0%{?fedora} || 0%{?rhel} > 5
+# NOTE: do **NOT** change 'epel' to 'rhel' here, as this spec is also
+# used to do RHEL builds without EPEL
+%if 0%{?fedora} || 0%{?epel} > 5
 # If the definition isn't available for python3_pkgversion, define it
 %{?!python3_pkgversion:%global python3_pkgversion 3}
 %bcond_without  python3