add option to make chunksize adaptive for multiprocessing.pool methods

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-16.14:56:42.964>
labels = ['type-feature', 'library']
title = 'add option to make chunksize adaptive for multiprocessing.pool methods'
updated_at = <Date 2020-01-16.18:07:25.058>
user = 'https://github.com/fgregg'

bugs.python.org fields:

activity = <Date 2020-01-16.18:07:25.058>
actor = 'pablogsal'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-01-16.14:56:42.964>
creator = 'fgregg'
dependencies = []
files = []
hgrepos = []
issue_num = 39362
keywords = []
message_count = 1.0
messages = ['360126']
nosy_count = 4.0
nosy_names = ['pitrou', 'davin', 'pablogsal', 'fgregg']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39362'
versions = []