There is a small change in last line of the python code.

Updated code

from gremlin_python.driver import client

client = client.Client('ws://localhost:8182/gremlin', 'g')

mgmtScript = "mgmt = graph.openManagement()\n" + \
"if (mgmt.getPropertyKey('name') != null) return false\n" + \
"mgmt.makePropertyKey('name').dataType(String.class).make()\n" + \
"mgmt.makePropertyKey('vl_prop').dataType(Float.class).cardinality(LIST).make()\n" +\
"mgmt.commit()\n" + \
"return true";

client.submit(mgmtScript).next()

f = open("only_vertex.txt", "r")

create_query = f.read()

client.submit(create_query).next()