tests: cleanup test code

- remove unused imports in test cases
- fix code smells and code style in conftest.py

test/test_ipv6/test_ipv6.py

@@ -1,6 +1,3 @@
-import pytest
-
-
 def test_unknown_virtual_host_ipv4(docker_compose, nginxproxy):
     r = nginxproxy.get("http://nginx-proxy/port")
     assert r.status_code == 503

test/test_ipv6/test_ipv6_prefer_ipv4_network.py

@@ -1,6 +1,3 @@
-import pytest
-
-
 def test_forwards_to_ipv4_only_network(docker_compose, nginxproxy):
     r = nginxproxy.get("http://ipv4only.nginx-proxy.tld/port")
     assert r.status_code == 200   

test/test_ipv6/test_ipv6_prefer_ipv6_network.py

@@ -1,6 +1,3 @@
-import pytest
-
-
 def test_forwards_to_ipv4_only_network(docker_compose, nginxproxy):
     r = nginxproxy.get("http://ipv4only.nginx-proxy.tld/port")
     assert r.status_code == 200