Solve the programming task below in a Python markdown code block.
According to ISO 8601, the first calendar week (1) starts with the week containing the first thursday in january.
Every year contains of 52 (53 for leap years) calendar weeks.

**Your task is** to calculate the calendar week (1-53) from a given date.
For example, the calendar week for the date `2019-01-01` (string) should be 1 (int).

Good luck 👍

See also [ISO week date](https://en.wikipedia.org/wiki/ISO_week_date) and [Week Number](https://en.wikipedia.org/wiki/Week#Week_numbering) on Wikipedia for further information about calendar weeks.

On [whatweekisit.org](http://whatweekisit.org/) you may click through the calender and study calendar weeks in more depth.

*heads-up:* `require(xxx)` has been disabled


Thanks to @ZED.CWT, @Unnamed and @proxya for their feedback.
Also feel free to reuse/extend the following starter code:
```python
def get_calendar_week(date_string):
```