Shared interface to IDNA encoding and decoding, using the 
idna PyPI package if available, otherwise the stdlib 
implementation.
| Function | _idnaBytes | Convert some text typed by a human into some ASCII bytes. | 
| Function | _idnaText | Convert some IDNA-encoded octets into some human-readable text. | 
Convert some text typed by a human into some ASCII bytes.
This is provided to allow us to use the partially-broken 
IDNA implementation in the standard library if the more-correct idna package is 
not available; service_identity is somewhat stricter about 
this.
| Parameters | text | A domain name, hopefully. (type: unicode) | 
| Returns | The domain name's IDNA representation, encoded as bytes. (type: bytes) | |