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ConnectionRefusedError exceptions wrapped by urllib.error.URLError.

This change should fix sporadic failures in test_urllib2net.

Patch by Buck Golemon.

Original patch by Tal Einat.

Patch by Févry Thibault.

revised by R. David Murray.

Report and original fix by Vajrasky Kok.

Before, if a default was set on the parent parser, any default for that
variable set via set_defaults on a subparser would be ignored.  Now
the subparser set_defaults is honored.

Patch by Jyrki Pullianinen.

It is unlikely anyone is using the fact that the dictionary returned
by the 'params' attribute was previously writable, but even if someone
is the API is provisional so this kind of change is acceptable (and
needed, to get the API "right" before it becomes official).

Patch by Stéphane Wirtel.

Issue #22653: Fix an assertion failure in debug mode when doing a reentrant dict insertion in debug mode.