How to sweep a segwit bech32 private key?
Use Electrum 3.1.1 and prepend "p2wpkh:" to the private key before importing or sweeping it. For example:

p2wpkh:5Kkzs8XrJNAmf9VQDFeGBfaRvSByAvPK6DbDXw5BVqswWaXSG2Y