Add dependency on python3-urlgrabber

Starting with urlgrabber 4.0, this script is only shipped with the
python3 variant.  However, yum only pulls in the python2 variant and
thus doesn't get the script.

While a bit hacky, let's make yum depend on python3-urlgrabber in this
limited time window between now and F30 EOL during which yum is still
available in the repos and thus should work.

yum.spec

@@ -64,7 +64,7 @@ BuildRequires: bash-completion
 Summary: RPM package installer/updater/manager
 Name: yum
 Version: 3.4.3
-Release: 521%{?dist}
+Release: 522%{?dist}
 License: GPLv2+
 Source0: http://yum.baseurl.org/download/3.4/%{name}-%{version}.tar.gz
 Source1: yum.conf.fedora
@@ -100,6 +100,8 @@ Requires: python2 >= 2.4
 Requires: python2-rpm, rpm >= 0:4.4.2
 Requires: python2-iniparse
 Requires: python2-urlgrabber >= 3.9.0-8
+# This is needed in order to get the /usr/libexec/urlgrabber-ext-down script
+Requires: python3-urlgrabber >= 4.0.0
 Requires: yum-metadata-parser >= 1.1.0
 Requires: python2-gpg
 # rawhide is >= 0.5.3-7.fc18 ... as this is added.
@@ -496,6 +498,9 @@ exit 0
 %endif
 
 %changelog
+* Tue May 21 2019 Michal Domonkos <mdomonko@redhat.com.com> - 3.4.3-522
+- Add dependency on python3-urlgrabber (for /usr/libexec/urlgrabber-ext-down)
+
 * Mon Feb 11 2019 Kevin Fenzi <kevin@scrye.com> - 3.4.3-521
 - Fix FTBFS by explicitly using python2. Fixes bug #1606775