[FIX] hr_timesheet_sheet: week number according to ISO 8601

The ISO weekday should be computed with isocalendar dateformat
The exact directive is '%V' according to GNU/Linux system strftime
http://man7.org/linux/man-pages/man3/strftime.3.html

However '%V' is not documented in official cross-platform syntax supported on
all python built, according to standardized by ANSI C formats
https://docs.python.org/2.7/library/datetime.html#strftime-and-strptime-behavior

as '%V' may not be supported on other platforms such as MS Windows, use the
cross-platform 'isoformat()' method.

Note that according to ISO 8601, the start and end of date may belong to the
previous or next year.
e.g.
>>> datetime.datetime.strptime('2005-01-01', '%Y-%m-%d').isocalendar()
(2004, 53, 6)
>>> datetime.datetime.strptime('2005-01-01', '%Y-%m-%d').strftime('%W')
'00'

Fixes #8927

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