<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def list_pull_requests(self, username, page, status=None):
 """ List pull-requests filed by user. Params: username (string):
 filters the username of the user whose activity you are interested in. page (integer):
 the page requested. Defaults to 1. status (string):
 filter the status of pull requests. Default: Open, can be Closed, Merged, All. Returns: list: A list of Pull-Requests filed by a given user for all the projects for given Pagure instance. """

        request_url = "{}/api/0/user/{}/requests/filed".format(self.instance, username)

        payload = {}
        if username is not None:
            payload['username'] = username
        if page is not None:
            payload['page'] = page
        if status is not None:
            payload['status'] = status

        return_value = self._call_api(request_url, params=payload)

        return return_value['requests']