devel/cmake-doc: fix build with python3.6 default

File "/wrkdirs/usr/ports/devel/cmake-doc/work/cmake-3.14.1/Utilities/Sphinx/create_identifiers.py", line 14, in <module>

File "/usr/local/lib/python3.6/encodings/ascii.py", line 26, in decode
  return codecs.ascii_decode(input, self.errors)[0]

UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 38147: ordinal not in range(128)

PR: 234633
Reported by: antoine