It appears I also have the following error :
# UserWarning: Cannot access C:/Users/thomas/Documents\.credentials\sheets.googleapis.com-python-quickstart.json: No such file or directory, at line 255, in "C:\Program Files\Autodesk\Maya2018\plug-ins\MASH\scripts\oauth2client\_helpers.py"
I made a .credential in the Documents folder as mentioned (also into the Users and Users/myName) but nothing to do.
It is actually also looking for a file called sheets.googleapis.com-python-quickstart.json which I have no idea where it should come from...
As the quickstart page mentions I made a client_secret.json which I drop in the same folder as the quickstart.py script. Also Installed the Google Client Lib with the command mentioned which worked a charm.
Really no success to run the script and always the same issues.
From 3dsmax 2017 it is worse, it can't even load the quickstart.py script, and can't find most of the headers. (at least Maya 2018 loads it without error).