ThreadPoolExecutor and ProcessPoolExecutor, dynamic worker count

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-02-27.11:40:15.444>
labels = ['3.8', 'library']
title = 'ThreadPoolExecutor and ProcessPoolExecutor, dynamic worker count'
updated_at = <Date 2019-02-27.11:40:15.444>
user = 'https://bugs.python.org/FabianDill'

bugs.python.org fields:

activity = <Date 2019-02-27.11:40:15.444>
actor = 'Fabian Dill'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-02-27.11:40:15.444>
creator = 'Fabian Dill'
dependencies = []
files = []
hgrepos = []
issue_num = 36133
keywords = []
message_count = 1.0
messages = ['336745']
nosy_count = 1.0
nosy_names = ['Fabian Dill']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue36133'
versions = ['Python 3.8']