Notice handler fails when notice contains utf8 bytes

[asterite3]

Notice can arive from server as utf8 and it's decoding as ascii can cause and error, for example:

From cffi callback <function <lambda> at 0x0000000001dbab60>:
Traceback (most recent call last):
  File "psycopg2cffi/_impl/connection.py", line 132, in <lambda>
    arg, bytes_to_ascii(ffi.string(message))))
  File "psycopg2cffi/_impl/adapters.py", line 314, in bytes_to_ascii
    return b.decode('ascii')
UnicodeDecodeError: ('ascii', '...

Same issue as https://github.com/chtd/psycopg2cffi/issues/56

[lopuhin]

Thanks @asterite3 👍