Provide a User-Agent string in delphi_epidata.py
Provide a User-Agent string in delphi_epidata.py

This will help us track what proportion of API users are from each
client.

cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata