hopefully fix deprecation warning

[bt2901]

Apparently, the function used in protobuf code was deprecated:
https://python.readthedocs.io/en/latest/whatsnew/3.6.html#deprecated-functions-and-types-of-the-c-api

which leads to the following warnings (see #975):

DeprecationWarning: PyUnicode_AsEncodedObject() is deprecated; use PyUnicode_AsEncodedString() to encode from str to bytes or PyCodec_Encode() for generic encoding

I think I found the sole line responsible and the fix is really simple, but I'm out of my depth here; could someone please check that everything works and backward-compatible?