Allow easy opt-out from tests using network services

[ptrcnull]

in some environments, like building packages for Linux distributions, external network connections are not allowed

right now, python-future has two tests that perform network connections:

• tests/test_future/test_standard_library.py::TestStandardLibraryReorganization::test_moves_urllib_request_http
• tests/test_future/test_standard_library.py::TestStandardLibraryReorganization::test_urllib_request_http

it would be nice to provide an easy way to disable those tests, e.g. with an environmental variable ( see https://github.com/pypa/distlib/blob/32301789a7815de0e74f57fe013ae52af717a3da/tests/test_index.py#L343 )

[ptrcnull]

ah, there's more tests, like tests/test_future/test_urllibnet.py - but those actually skip on Resource not available errors