I have an older machine which has both Python 2.7.14 & 3.5.3 installed.
Trying to build libxcb with the current setting requiring a minimum of Python 2.6 made configure choose Python 2.7.14, which failed with:
File "./c_client.py", line 2270 key = (*self.name[:-1], field.enum) ^ SyntaxError: invalid syntax
Raising the minimum to 3.0 made it instead pick 3.5.3 which worked fine. I've not tested with 3.0 or anything below 3.5.3, but have chosen 3.0 based on the version shown in PEP 3132 – Extended Iterable Unpacking.