padding: 3 causes crash

[MKaganUlker]

trafficstars

padding: 3 line here gives an error. i have to delete it every time i import kivy

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• [ ] Title is descriptive/clear for inclusion in release notes.
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[akshayaurora]

Please re-open with a proper description of what this pr fixes, hopefully with a issue that explains in details and with a minimal code that can reproduce the issue.

[kuzeyron]

Since TabbedPanel is based on GridLayout. The padding is supposed to be written like this: padding: 3, 0 or padding: 3, 0, 3, 0.