Add contextlib.convert_exception manager

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-06-28.15:48:31.169>
labels = ['3.7', 'type-feature', 'library']
title = 'Add contextlib.convert_exception manager'
updated_at = <Date 2017-06-29.02:43:10.856>
user = 'https://github.com/stevendaprano'

bugs.python.org fields:

activity = <Date 2017-06-29.02:43:10.856>
actor = 'ncoghlan'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-06-28.15:48:31.169>
creator = 'steven.daprano'
dependencies = []
files = []
hgrepos = []
issue_num = 30792
keywords = []
message_count = 2.0
messages = ['297193', '297245']
nosy_count = 2.0
nosy_names = ['ncoghlan', 'steven.daprano']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30792'
versions = ['Python 3.7']