BaseProxy cache should be cleaned when Manager client is reconnected

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-09-21.15:48:45.085>
labels = ['3.7', 'type-bug', 'library']
title = 'BaseProxy cache should be cleaned when Manager client is reconnected'
updated_at = <Date 2019-05-06.19:54:38.686>
user = 'https://github.com/wynfred'

bugs.python.org fields:

activity = <Date 2019-05-06.19:54:38.686>
actor = 'wynfred'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-09-21.15:48:45.085>
creator = 'wynfred'
dependencies = []
files = []
hgrepos = []
issue_num = 34766
keywords = ['patch']
message_count = 5.0
messages = ['326388', '329480', '331765', '333896', '341631']
nosy_count = 3.0
nosy_names = ['pitrou', 'davin', 'wynfred']
pr_nums = ['9476']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34766'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7']