Add xml.tool to pretty print XML like json.tool

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-08-24.14:55:33.370>
labels = ['type-feature', 'library', '3.9']
title = 'Add xml.tool to pretty print XML like json.tool'
updated_at = <Date 2019-08-25.13:24:16.646>
user = 'https://github.com/tirkarthi'

bugs.python.org fields:

activity = <Date 2019-08-25.13:24:16.646>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-08-24.14:55:33.370>
creator = 'xtreak'
dependencies = []
files = []
hgrepos = []
issue_num = 37940
keywords = []
message_count = 6.0
messages = ['350372', '350377', '350440', '350441', '350445', '350454']
nosy_count = 5.0
nosy_names = ['rhettinger', 'scoder', 'eli.bendersky', 'serhiy.storchaka', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37940'
versions = ['Python 3.9']