Factor out / add bdb.Bdb.is_skipped_frame

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-18.03:30:29.652>
labels = ['3.8', 'type-feature', 'library']
title = 'Factor out / add bdb.Bdb.is_skipped_frame'
updated_at = <Date 2019-03-18.03:31:28.370>
user = 'https://github.com/blueyed'

bugs.python.org fields:

activity = <Date 2019-03-18.03:31:28.370>
actor = 'blueyed'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-03-18.03:30:29.652>
creator = 'blueyed'
dependencies = []
files = []
hgrepos = []
issue_num = 36335
keywords = ['patch']
message_count = 1.0
messages = ['338159']
nosy_count = 1.0
nosy_names = ['blueyed']
pr_nums = ['12392']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36335'
versions = ['Python 3.8']