<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_issns_for_journal(nlm_id):
 """Get a list of the ISSN numbers for a journal given its NLM ID. Information on NLM XML DTDs is available at https://www.nlm.nih.gov/databases/dtd/ """

    params = {'db': 'nlmcatalog',
              'retmode': 'xml',
              'id': nlm_id}
    tree = send_request(pubmed_fetch, params)
    if tree is None:
        return None
    issn_list = tree.findall('.//ISSN')
    issn_linking = tree.findall('.//ISSNLinking')
    issns = issn_list + issn_linking
    # No ISSNs found!
    if not issns:
        return None
    else:
        return [issn.text for issn in issns]