run pool.submit in callback when future.set_result

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-07-29.03:25:08.090>
labels = ['type-bug', 'library']
title = 'run pool.submit in callback when future.set_result'
updated_at = <Date 2018-07-29.03:25:08.090>
user = 'https://github.com/gaoxinge'

bugs.python.org fields:

activity = <Date 2018-07-29.03:25:08.090>
actor = 'gaoxinge'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-07-29.03:25:08.090>
creator = 'gaoxinge'
dependencies = []
files = []
hgrepos = []
issue_num = 34268
keywords = []
message_count = 1.0
messages = ['322605']
nosy_count = 1.0
nosy_names = ['gaoxinge']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34268'
versions = ['Python 3.6']