Format Pandas datetime column as year-week

Question

I have data in a CSV file formatted as YYYY-UU (e.g., 2015-52). I am trying to import that data into a pandas dataframe, but I get an incorrect format.

dateparse = lambda dates: pd.datetime.strptime(dates, '%Y-%U')
df = pd.read_csv('weekly.csv', parse_dates=True, index_col='date', date_parser=dateparse)

df.head()

            cnt
date    
2013-01-01  214567
2013-01-01  200239
2013-01-01  218296

I need the column date format to be something like this:

        cnt
date    
2013-01 60000
2013-02 62000
2013-03 65000

...