Allows to read package version without installing dependencies.
```
import eventlet
print(eventlet.__version__)  # ok
eventlet.sleep()  # error
```

Previous PropagateError.__init__() failed to call Exception.__init__() at all.
Moreover, as described in https://bugs.python.org/issue1692335, setting
self.args is important for an Exception subclass with nonstandard constructor
arguments.

When an Exception subclass sets self.args, it makes the subclass pickleable --
but instead of str(instance) returning the string passed to
Exception.__init__(), it instead returns the string representation of
self.args. So capture the message string, and in addition to passing it to
Exception.__init__(), also override __str__() and return the same string.

https://github.com/eventlet/eventlet/pull/359

Introduced in #309, this enumeration should be over the items of the
dict in order to work correctly.

https://github.com/eventlet/eventlet/pull/356
https://github.com/eventlet/eventlet/pull/309

When calling getaddrinfo with an unspecified address family, we:

 * try to get an IPv6 address for the host, first trying /etc/hosts and
   failing that, using any external nameserver, then
 * try to get an IPv4 address for it, with the same /etc/hosts ->
   external nameserver priority.

Note that if the host can be resolved using /etc/hosts but only to one
of the two address families, we still attempt to contact the external
nameserver to resolve it in the other family.

Previously, if this timed out or raised a DNSException other than
NXDOMAIN, EAI_EAGAIN_ERROR or EAI_NODATA_ERROR would be raised even
though we could resolve the host. Now, those errors will only be raised
if neither attempt at resolution succeeded.

https://github.com/eventlet/eventlet/pull/354