add python3 to image (needed for vp scripts)

var/local/mkosi/mkosi.postinst

@@ -14,6 +14,8 @@ yum install -y --releasever=7 \
     fetch-crl \
     gdb \
     yum-utils \
-    python-requests
+    python-requests \
+    python3 \
+    python3-requests
 
 yum --releasever=7 --disablerepo="*" --enablerepo="osg-upcoming-testing" install -y xrootd-rucioN2N-for-Xcache