pickle: respect dispatch for functions again

GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = None
created_at = <Date 2016-05-05.00:13:46.295>
labels = ['extension-modules', 'type-feature', '3.7']
title = 'pickle: respect dispatch for functions again'
updated_at = <Date 2018-01-08.12:52:28.347>
user = 'https://github.com/torkve'

bugs.python.org fields:

activity = <Date 2018-01-08.12:52:28.347>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2016-05-05.00:13:46.295>
creator = 'torkve'
dependencies = []
files = ['42727', '42751']
hgrepos = []
issue_num = 26959
keywords = ['patch']
message_count = 5.0
messages = ['264866', '264950', '264958', '279809', '288153']
nosy_count = 5.0
nosy_names = ['pitrou', 'hniksic', 'alexandre.vassalotti', 'serhiy.storchaka', 'torkve']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue26959'
versions = ['Python 3.7']