make it simpler to round fractions (support different rounding modes for division)

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-11-08.10:13:02.851>
labels = ['3.7', 'type-feature', 'library']
title = 'make it simpler to round fractions'
updated_at = <Date 2017-11-09.08:45:06.327>
user = 'https://github.com/wm75'

bugs.python.org fields:

activity = <Date 2017-11-09.08:45:06.327>
actor = 'wolma'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-11-08.10:13:02.851>
creator = 'wolma'
dependencies = []
files = ['47254']
hgrepos = []
issue_num = 31978
keywords = ['patch']
message_count = 11.0
messages = ['305817', '305818', '305843', '305844', '305846', '305851', '305879', '305941', '305942', '305943', '305944']
nosy_count = 8.0
nosy_names = ['rhettinger', 'facundobatista', 'mark.dickinson', 'belopolsky', 'stutzbach', 'skrah', 'serhiy.storchaka', 'wolma']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31978'
versions = ['Python 3.7']