fix: Ensure year is always 4 digits

[Darkheir]

Because of this issue in CPython on some platform the dates having a year smaller than 1000 will be on less than 4 digits.

format_datetime(datetime(200, 1, 1)) # 200-01-01T00:00:00Z

This PR make sure that the year is always formatted with 4 digits:

format_datetime(datetime(200, 1, 1)) # 0200-01-01T00:00:00Z

[CLAassistant]

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