bdb and pdb: Add watchpoint function

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-05-22.12:09:45.458>
labels = ['3.7', 'type-feature', 'library']
title = 'bdb and pdb: Add watchpoint function'
updated_at = <Date 2017-06-28.09:37:10.680>
user = 'https://github.com/mlouielu'

bugs.python.org fields:

activity = <Date 2017-06-28.09:37:10.680>
actor = 'xdegaye'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-05-22.12:09:45.458>
creator = 'louielu'
dependencies = []
files = []
hgrepos = []
issue_num = 30429
keywords = []
message_count = 4.0
messages = ['294140', '294230', '296872', '297167']
nosy_count = 3.0
nosy_names = ['barry', 'xdegaye', 'louielu']
pr_nums = ['1756']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30429'
versions = ['Python 3.7']