Shelve should default to the default Pickle protocol instead of hardcoding version 3

[marco-c]

BPO	42071
Nosy	@rhettinger, @ZackerySpytz, @marco-c
PRs	bpo-34204: Make the shelve library use Pickle's default protocol version instead of defaulting to version 3 #22751
Superseder	bpo-34204: Bump the default pickle protocol in shelve

^{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 2020-10-18.19:08:59.829>
created_at = <Date 2020-10-18.18:58:45.208>
labels = ['type-feature', 'library', '3.10']
title = 'Shelve should default to the default Pickle protocol instead of hardcoding version 3'
updated_at = <Date 2020-10-18.19:10:43.614>
user = 'https://github.com/marco-c'

bugs.python.org fields:

activity = <Date 2020-10-18.19:10:43.614>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2020-10-18.19:08:59.829>
closer = 'ZackerySpytz'
components = ['Library (Lib)']
creation = <Date 2020-10-18.18:58:45.208>
creator = 'marco-c'
dependencies = []
files = []
hgrepos = []
issue_num = 42071
keywords = ['patch']
message_count = 3.0
messages = ['378885', '378887', '378888']
nosy_count = 3.0
nosy_names = ['rhettinger', 'ZackerySpytz', 'marco-c']
pr_nums = ['22751']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '34204'
type = 'enhancement'
url = 'https://bugs.python.org/issue42071'
versions = ['Python 3.10']

[marco-c]

Shelve is currently defaulting to Pickle protocol 3, instead of using Pickle's default protocol for the Python version in use.

This way, Shelve's users don't benefit from improvements introduced in newer Pickle protocols, unless they notice it and manually pass a newer protocol version to shelve.open or the Shelf constructor.

[ZackerySpytz]

Unfortunately, this is a duplicate of bpo-34204 (which already has a pull request).

[rhettinger]

This seems reasonable to me.

There might be some issue is existing code where different versions of Python are sharing the same shelf, but I don't think this is common.