`bytes_concat` and Buffer cleanup

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-03-24.01:48:29.170>
labels = ['expert-C-API', '3.9', '3.10']
title = '`bytes_concat` and Buffer cleanup'
updated_at = <Date 2021-03-24.01:48:29.170>
user = 'https://github.com/seberg'

bugs.python.org fields:

activity = <Date 2021-03-24.01:48:29.170>
actor = 'seberg'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['C API']
creation = <Date 2021-03-24.01:48:29.170>
creator = 'seberg'
dependencies = []
files = []
hgrepos = []
issue_num = 43608
keywords = []
message_count = 1.0
messages = ['389428']
nosy_count = 1.0
nosy_names = ['seberg']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43608'
versions = ['Python 3.9', 'Python 3.10']