Deprecate unbound super methods

[stevendaprano]

BPO	37808
Nosy	@gvanrossum, @stevendaprano, @ZackerySpytz
PRs	python/cpython#15205

^{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: ```python assignee = None closed_at = None created_at = labels = ['3.9'] title = 'Deprecate unbound super methods' updated_at = user = 'https://github.com/stevendaprano' ``` bugs.python.org fields: ```python activity = actor = 'gvanrossum' assignee = 'none' closed = False closed_date = None closer = None components = [] creation = creator = 'steven.daprano' dependencies = [] files = [] hgrepos = [] issue_num = 37808 keywords = ['patch'] message_count = 2.0 messages = ['349338', '349405'] nosy_count = 3.0 nosy_names = ['gvanrossum', 'steven.daprano', 'ZackerySpytz'] pr_nums = ['15205'] priority = 'normal' resolution = None stage = 'patch review' status = 'open' superseder = None type = None url = 'https://bugs.python.org/issue37808' versions = ['Python 3.9'] ```

[stevendaprano]

As per the discussion here, let's deprecate unbound super methods.

https://discuss.python.org/t/is-it-time-to-deprecate-unbound-super-methods/1833

[gvanrossum]

I don't have time to review this, sorry.