ast.unparse: Simplify buffering logic

[isidentical]

BPO	43417
Nosy	@isidentical
PRs	bpo-43417: Better buffer handling for ast.unparse #24772

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/isidentical'
closed_at = <Date 2021-05-08.23:33:03.577>
created_at = <Date 2021-03-06.11:55:57.699>
labels = ['library', '3.10']
title = 'ast.unparse: Simplify buffering logic'
updated_at = <Date 2021-05-08.23:33:03.577>
user = 'https://github.com/isidentical'

bugs.python.org fields:

activity = <Date 2021-05-08.23:33:03.577>
actor = 'BTaskaya'
assignee = 'BTaskaya'
closed = True
closed_date = <Date 2021-05-08.23:33:03.577>
closer = 'BTaskaya'
components = ['Library (Lib)']
creation = <Date 2021-03-06.11:55:57.699>
creator = 'BTaskaya'
dependencies = []
files = []
hgrepos = []
issue_num = 43417
keywords = ['patch']
message_count = 1.0
messages = ['388197']
nosy_count = 1.0
nosy_names = ['BTaskaya']
pr_nums = ['24772']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue43417'
versions = ['Python 3.10']

[isidentical]

Currently, buffer is just an instance-level list that is used in various places to avoid directly writing stuff into the real source buffer, though the design is pretty complicated and hard to use.

There are various use cases (like omitting the empty space when unparsing argument-less lambdas, e.g: lambda : 2 + 2) we could've use this buffer system if it was offering a stackable version (like multiple levels of buffers).

What I think is we should probably do this with a proper context manager and in the context capture all writings into a list where we would return after the context is closed;

with self.buffered() as buffer:
    self._write_fstring_inner(node)
return self._write_str_avoiding_backslashes("".join(buffer))