tests: Actually check for network in @requires_network
tests: Actually check for network in @requires_network

This is needed because @requires_network:
- was too tightly coupled with the kind of exceptions a test might
  raise,
- needed an errno to check against, but this errno is not always
  available. An example is
  test/with_dummyserver/test_proxy_poolmanager.py:TestHTTPProxyManager.test_https_proxy_timeout
  where only a string representation is available in exception.

This check still assumes that TARPIT_HOST is reachable iif there is a
route to Internet.

urllib3/urllib3,sigmavirus24/urllib3,Lukasa/urllib3,urllib3/urllib3,Disassem/urllib3,Disassem/urllib3,Lukasa/urllib3,sigmavirus24/urllib3