Accept file mode "rw" to open-or-create a file and seek to the start

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-05.04:36:25.742>
labels = ['type-feature']
title = 'Accept file mode "rw" to open-or-create a file and seek to the start'
updated_at = <Date 2021-02-05.17:49:28.700>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2021-02-05.17:49:28.700>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-02-05.04:36:25.742>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 43133
keywords = []
message_count = 2.0
messages = ['386510', '386533']
nosy_count = 2.0
nosy_names = ['ncoghlan', 'serhiy.storchaka']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43133'
versions = []