Add deferred single-threaded/fake executor to concurrent.futures

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-22.01:39:20.835>
labels = ['type-feature', '3.7']
title = 'Add deferred single-threaded/fake executor to concurrent.futures'
updated_at = <Date 2020-05-22.14:18:17.279>
user = 'https://bugs.python.org/BrianMcCutchon'

bugs.python.org fields:

activity = <Date 2020-05-22.14:18:17.279>
actor = 'santagada'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-03-22.01:39:20.835>
creator = 'Brian McCutchon'
dependencies = []
files = []
hgrepos = []
issue_num = 36395
keywords = []
message_count = 9.0
messages = ['338576', '341616', '341624', '341625', '341660', '341740', '341797', '341890', '369603']
nosy_count = 4.0
nosy_names = ['bquinlan', 'pitrou', 'santagada', 'Brian McCutchon']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36395'
versions = ['Python 3.7']