Merge branch 'backport/3.6/testing/pytest-xfail-strict' into 3.6

This change makes pytest treat XPASS as failures.

This is per the best practices outlined by Paul Ganssle in his
lightning talk https://pganssle-talks.github.io/xfail-lightning

pytest.ci.ini

@@ -7,3 +7,4 @@ junit_suite_name = aiohttp_test_suite
 norecursedirs = dist docs build .tox .eggs
 minversion = 3.8.2
 testpaths = tests/
+xfail_strict = true

pytest.ini

@@ -7,3 +7,4 @@ junit_suite_name = aiohttp_test_suite
 norecursedirs = dist docs build .tox .eggs
 minversion = 3.8.2
 testpaths = tests/
+xfail_strict = true

tests/test_web_protocol.py

@@ -1,7 +1,6 @@
 # Tests for aiohttp/server.py
 
 import asyncio
-import platform
 import socket
 from functools import partial
 from unittest import mock
@@ -10,8 +9,6 @@
 
 from aiohttp import helpers, http, streams, web
 
-IS_MACOS = platform.system() == 'Darwin'
-
 
 @pytest.fixture
 def make_srv(loop, manager):
@@ -341,11 +338,6 @@ def close():
         "Error handling request", exc_info=mock.ANY)
 
 
-@pytest.mark.xfail(
-    IS_MACOS,
-    raises=TypeError,
-    reason='Intermittently fails on macOS',
-)
 async def test_handle_uncompleted_pipe(
         make_srv, transport, request_handler, handle_with_error):
     closed = False