Allow `ast.unparse` to handle NaNs and empty sets

[Kodiologist]

BPO	43521
Nosy	@rhettinger, @Kodiologist, @miss-islington, @isidentical
PRs	bpo-43521: Allow ast.unparse with empty sets and NaN #24897 [3.9] bpo-43521: Allow ast.unparse with empty sets and NaN (GH-24897) #24921

^{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 = None
closed_at = <Date 2021-03-18.22:52:29.941>
created_at = <Date 2021-03-16.19:39:06.673>
labels = ['type-feature', '3.9', '3.10']
title = 'Allow `ast.unparse` to handle NaNs and empty sets'
updated_at = <Date 2021-03-18.22:52:29.940>
user = 'https://github.com/Kodiologist'

bugs.python.org fields:

activity = <Date 2021-03-18.22:52:29.940>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-03-18.22:52:29.941>
closer = 'pablogsal'
components = []
creation = <Date 2021-03-16.19:39:06.673>
creator = 'Kodiologist'
dependencies = []
files = []
hgrepos = []
issue_num = 43521
keywords = ['patch']
message_count = 4.0
messages = ['388872', '388874', '389032', '389046']
nosy_count = 4.0
nosy_names = ['rhettinger', 'Kodiologist', 'miss-islington', 'BTaskaya']
pr_nums = ['24897', '24921']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43521'
versions = ['Python 3.9', 'Python 3.10']

[Kodiologist]

ast.unparse throws an error on an empty set, and it produces nan for NaN, which isn't a legal Python literal.

PR to follow shortly.

[isidentical]

The reason that we weren't support these cases was there were simply no way achieve them by parsing code so we simply ignored (empty sets etc). Though considering that you have a decent use case in hy, I'd agree that these small additions are viable.

[miss-islington]

New changeset 08ff436 by Kodi Arfer in branch 'master':
bpo-43521: Allow ast.unparse with empty sets and NaN (GH-24897)
08ff436

[miss-islington]

New changeset e8e3419 by Miss Islington (bot) in branch '3.9':
bpo-43521: Allow ast.unparse with empty sets and NaN (GH-24897)
e8e3419