Warn when __loader__ != __spec__.loader

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-09-22.18:42:23.496>
labels = ['3.7', 'type-feature', 'library']
title = 'Warn when __loader__ != __spec__.loader'
updated_at = <Date 2017-12-13.15:57:43.686>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2017-12-13.15:57:43.686>
actor = 'eric.smith'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-09-22.18:42:23.496>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 31554
keywords = []
message_count = 1.0
messages = ['302764']
nosy_count = 5.0
nosy_names = ['barry', 'brett.cannon', 'ncoghlan', 'eric.smith', 'eric.snow']
pr_nums = []
priority = 'low'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31554'
versions = ['Python 3.7']