Support default env credentials (e.g. needed in Google Colaboratory (Colab)) + workaround for now

FileNotFoundError                         Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/oauth2client/clientsecrets.py in _loadfile(filename)
    120     try:
--> 121         with open(filename, 'r') as fp:
    122             obj = json.load(fp)

FileNotFoundError: [Errno 2] No such file or directory: 'client_secrets.json'

During handling of the above exception, another exception occurred:

InvalidClientSecretsError                 Traceback (most recent call last)
9 frames
/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in LoadClientConfigFile(self, client_config_file)
    476             client_type, client_info = clientsecrets.loadfile(
--> 477                 client_config_file
    478             )

/usr/local/lib/python3.7/dist-packages/oauth2client/clientsecrets.py in loadfile(filename, cache)
    164     if not cache:
--> 165         return _loadfile(filename)
    166 

/usr/local/lib/python3.7/dist-packages/oauth2client/clientsecrets.py in _loadfile(filename)
    124         raise InvalidClientSecretsError('Error opening file', exc.filename,
--> 125                                         exc.strerror, exc.errno)
    126     return _validate_clientsecrets(obj)

InvalidClientSecretsError: ('Error opening file', 'client_secrets.json', 'No such file or directory', 2)

During handling of the above exception, another exception occurred:

InvalidConfigError                        Traceback (most recent call last)
<ipython-input-3-e7c1a9729564> in <module>()
     16 uploaded = drive.CreateFile({'title': 'Sample file.txt'})
     17 uploaded.SetContentString('Sample upload file content')
---> 18 uploaded.Upload()
     19 print('Uploaded file with ID {}'.format(uploaded.get('id')))

/usr/local/lib/python3.7/dist-packages/pydrive2/files.py in Upload(self, param)
    495                 self._FilesPatch(param=param)
    496         else:
--> 497             self._FilesInsert(param=param)
    498 
    499     def Trash(self, param=None):

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in _decorated(self, *args, **kwargs)
     58                 self.auth.ServiceAuth()
     59             else:
---> 60                 self.auth.LocalWebserverAuth()
     61 
     62         # Initialise service if not built yet.

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in _decorated(self, *args, **kwargs)
    122             self.LoadCredentials()
    123         if self.flow is None:
--> 124             self.GetFlow()
    125         if self.credentials is None:
    126             code = decoratee(self, *args, **kwargs)

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in GetFlow(self)
    565             config in self.client_config for config in self.CLIENT_CONFIGS_LIST
    566         ):
--> 567             self.LoadClientConfig()
    568         constructor_kwargs = {
    569             "redirect_uri": self.client_config["redirect_uri"],

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in LoadClientConfig(self, backend)
    454                 )
    455         if backend == "file":
--> 456             self.LoadClientConfigFile()
    457         elif backend == "settings":
    458             self.LoadClientConfigSettings()

/usr/local/lib/python3.7/dist-packages/pydrive2/auth.py in LoadClientConfigFile(self, client_config_file)
    478             )
    479         except clientsecrets.InvalidClientSecretsError as error:
--> 480             raise InvalidConfigError("Invalid client secrets file %s" % error)
    481         if client_type not in (
    482             clientsecrets.TYPE_WEB,

InvalidConfigError: Invalid client secrets file ('Error opening file', 'client_secrets.json', 'No such file or directory', 2)