add python-requests to xcache.singularity.def

xcache.singularity.def

@@ -41,6 +41,7 @@ Purpose Xcache-for-RUCIO
   yum install -y vomsxrd
   yum install -y gdb
   yum install -y yum-utils
+  yum install -y python-requests
   debuginfo-install -y xrootd-server xrootd-client xrootd
   mkdir -p /data