2to3 no-ops on embeddable distribution

[mrginglymus]

BPO	37654
Nosy	@mrginglymus
Superseder	bpo-45544: Close 2to3 issues and list them here

^{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 2021-10-20.22:58:02.021>
created_at = <Date 2019-07-22.19:17:59.211>
labels = ['3.7', '3.8', 'type-bug', 'expert-2to3', '3.9']
title = '2to3 no-ops on embeddable distribution'
updated_at = <Date 2021-10-20.22:58:02.018>
user = 'https://github.com/mrginglymus'

bugs.python.org fields:

activity = <Date 2021-10-20.22:58:02.018>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-20.22:58:02.021>
closer = 'iritkatriel'
components = ['2to3 (2.x to 3.x conversion tool)']
creation = <Date 2019-07-22.19:17:59.211>
creator = 'Bill Collins'
dependencies = []
files = []
hgrepos = []
issue_num = 37654
keywords = []
message_count = 1.0
messages = ['348304']
nosy_count = 1.0
nosy_names = ['Bill Collins']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = '45544'
type = 'behavior'
url = 'https://bugs.python.org/issue37654'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']

[mrginglymus]

Firstly, I'd acknowledge that expecting 2to3 to work on the embeddable distribution might be the problem, but the mode of failure is silent and delayed.

The problem is that 2to3 loads fix names by searching for files in a package that end in '.py' (https://github.com/python/cpython/blame/master/Lib/lib2to3/refactor.py#L30) which isn't much use on the embeddable distribution where everything is pre-comipled.

So, installing an sdist package that relies on 2to3 will succeed, but any attempt to then actually use the installed package will fail.

I'm not sure whether the correct behavior here might be to

1. Fix 2to3 to load from pyc files (explicitly enabling 2to3 based installation on embeddable)
2. Fix 2to3 to fail when no converters are found (or similar; explicitly *disabling* 2to3 based installation on embeddable).

Or something else entirely.