plistlib doesn't skip whitespace in XML format detection

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-06-19.23:18:35.173>
labels = ['3.8', 'type-bug', '3.7', '3.9']
title = "plistlib doesn't skip whitespace in XML format detection"
updated_at = <Date 2019-07-14.16:13:50.742>
user = 'https://bugs.python.org/shaneg'

bugs.python.org fields:

activity = <Date 2019-07-14.16:13:50.742>
actor = 'ronaldoussoren'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-06-19.23:18:35.173>
creator = 'shaneg'
dependencies = []
files = []
hgrepos = []
issue_num = 37344
keywords = ['patch']
message_count = 7.0
messages = ['346089', '346090', '347912', '347920', '347921', '347923', '347924']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'serhiy.storchaka', 'shaneg']
pr_nums = ['14266']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37344'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']