Don't require network to inspect tests

[mrocklin]

Previously the get_ipv6 function was called at import time in test_core.py
This stopped any tests from running if a network adapter was not
attached

The results of this function were used only in one test,
so we move this call within that test and skip if it fails

[jrbourbeau]

Thanks @mrocklin!

We also do something similar over in distributed/distributed/comm/tests/test_comms.py

distributed/distributed/comm/tests/test_comms.py

Lines 43 to 47 in 467bbc2

	EXTERNAL_IP4 = get_ip()
	if has_ipv6():
	with warnings.catch_warnings(record=True):
	warnings.simplefilter("always")
	EXTERNAL_IP6 = get_ipv6()

If you want to update there too, that'd be welcome. Though this PR is a definite improvement over our current behavior and can be merged as is too

[bnavigator]

pytest.skip does not have a reason kwarg. Just use raise pytest.skip("no network access")

[  346s]     @pytest.mark.asyncio
[  346s]     async def test_server_listen():
[  346s]         """
[  346s]         Test various Server.listen() arguments and their effect.
[  346s]         """
[  346s]         import socket
[  346s]     
[  346s]         try:
[  346s]             EXTERNAL_IP4 = get_ip()
[  346s]             if has_ipv6():
[  346s]                 EXTERNAL_IP6 = get_ipv6()
[  346s]         except socket.gaierror:
[  346s] >           raise pytest.skip(reason="no network access")
[  346s] E           TypeError: skip() got an unexpected keyword argument 'reason'
[  346s] 
[  346s] distributed/tests/test_core.py:199: TypeError

[jrbourbeau]

Thanks @bnavigator! See #4467