ipaddress: increment/decrement on IPv*Interface disregards bitmask prefix

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-04-30.11:15:38.662>
labels = ['3.8', 'library']
title = 'ipaddress: increment/decrement on IPv*Interface disregards bitmask prefix'
updated_at = <Date 2021-04-30.11:15:38.662>
user = 'https://github.com/katsel'

bugs.python.org fields:

activity = <Date 2021-04-30.11:15:38.662>
actor = 'katsel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-04-30.11:15:38.662>
creator = 'katsel'
dependencies = []
files = []
hgrepos = []
issue_num = 43986
keywords = []
message_count = 1.0
messages = ['392417']
nosy_count = 1.0
nosy_names = ['katsel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43986'
versions = ['Python 3.8']