cannotsend error using python bitcoin library to send bitcoin on testnet
The issue was that addr is being provided an invalid address - a Main Net address does not work on Test Net, and vice versa.  (From the edit revisions history, the code sample you provided was using bitcoin.wallet.CBitcoinAddress('1JwSSubhmg6iPtRjtyqhUYYH7bZg3Lfy1T') which would provide a Main Net address)

Replace the address being supplied in the addr variable with a legitimate Test Net address, and it should work.