ftp tests in test_urllib2net should skip on unreachable network

[carljm]

BPO	43564
Nosy	@orsenthil, @giampaolo, @carljm, @brandtbucher
PRs	bpo-43564: preserve original exception in args of FTP URLError #24938

^{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 = None
created_at = <Date 2021-03-19.21:21:09.420>
labels = ['type-bug', 'tests', '3.10']
title = 'ftp tests in test_urllib2net should skip on unreachable network'
updated_at = <Date 2021-06-23.19:18:02.469>
user = 'https://github.com/carljm'

bugs.python.org fields:

activity = <Date 2021-06-23.19:18:02.469>
actor = 'brandtbucher'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2021-03-19.21:21:09.420>
creator = 'carljm'
dependencies = []
files = []
hgrepos = []
issue_num = 43564
keywords = ['patch']
message_count = 2.0
messages = ['389115', '389116']
nosy_count = 4.0
nosy_names = ['orsenthil', 'giampaolo.rodola', 'carljm', 'brandtbucher']
pr_nums = ['24938']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43564'
versions = ['Python 3.10']

[carljm]

In general it seems the CPython test suite takes care to skip instead of failing networked tests when the network is unavailable (c.f. support.transient_internet test helper).

In this case of the 5 FTP tests in test_urllib2net (that is, test_ftp, test_ftp_basic, test_ftp_default_timeout, test_ftp_no_timeout, and test_ftp_timeout), even though they use support_transient_internet, they still fail if the network is unavailable.

The reason is that they make calls which end up raising an exception in the form URLError("ftp error: OSError(101, 'Network is unreachable')" -- the original OSError is flattened into the exception string message, but is otherwise not in the exception args. This means that transient_network does not detect it as a suppressable exception.

It seems like many uses of URLError in urllib pass the original OSError directly to URLError.__init__(), which means it ends up in args and the unwrapping code in transient_internet is able to find the original OSError. But the ftp code instead directly interpolates the OSError into a new message string.

[carljm]

Created a PR that fixes this by being more consistent in how urllib wraps network errors. If there are backward-compatibility concerns with this change, another option could be some really ugly regex-matching code in test.support.transient_internet.