"""Get the first and last day of the month for a given year and month string.

Args:
    year_month_str (str): A string representing the year and month in the format '%Y-%m'.

Returns:
    tuple: A tuple containing two datetime.date objects: the first day and the last day of the month.

Raises:
    ValueError: If the input string is not in the expected '%Y-%m' format.
"""
You should write code starting with:
from datetime import datetime
from dateutil.relativedelta import relativedelta

def get_first_last_day(year_month_str: str) -> tuple:
    # TODO: Implement this function
    pass

