Eliminate implicit __main__ relative imports

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-03-28.09:22:30.381>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'Eliminate implicit __main__ relative imports'
updated_at = <Date 2017-06-06.19:54:51.571>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2017-06-06.19:54:51.571>
actor = 'ronaldoussoren'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2017-03-28.09:22:30.381>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 29929
keywords = []
message_count = 7.0
messages = ['290689', '290691', '290693', '295077', '295175', '295228', '295298']
nosy_count = 4.0
nosy_names = ['barry', 'ronaldoussoren', 'ncoghlan', 'Rosuav']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29929'
versions = ['Python 3.7']