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* Ported the library to Google Sheets API v4.
This is a transition release. The v3-related code is untouched,
but v4 is used by default. It is encouraged to move to v4 since
the API is faster and has more features.
API v4 is a significant change from v3. Some methods are not
backward compatible, so there's no support for this compatibility
in gspread either.
These methods and properties are not supported in v4:
* `Spreadsheet.updated`
* `Worksheet.updated`
* `Worksheet.export()`
* `Cell.input_value`