Support CDATA by xml.etree.(c)ElementTree

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-05-10.09:51:15.787>
labels = ['expert-XML', 'type-feature', '3.8', '3.9']
title = 'Support CDATA by xml.etree.(c)ElementTree'
updated_at = <Date 2019-05-10.12:32:09.165>
user = 'https://bugs.python.org/PierrevandeLaar'

bugs.python.org fields:

activity = <Date 2019-05-10.12:32:09.165>
actor = 'scoder'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['XML']
creation = <Date 2019-05-10.09:51:15.787>
creator = 'Pierre van de Laar'
dependencies = []
files = []
hgrepos = []
issue_num = 36874
keywords = []
message_count = 2.0
messages = ['342067', '342072']
nosy_count = 2.0
nosy_names = ['scoder', 'Pierre van de Laar']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36874'
versions = ['Python 3.8', 'Python 3.9']