Ignore a warning added in Python 3.7.

Also set explicitly the warnings filter to default when running tests
to prevent unittest from stomping on our ignore filter.

.appveyor.yml

@@ -1,7 +1,7 @@
 environment:
 # websockets only works on Python >= 3.4.
   CIBW_SKIP: cp27-* cp33-*
-  CIBW_TEST_COMMAND: python -m unittest websockets
+  CIBW_TEST_COMMAND: python -W default -m unittest websockets
   WEBSOCKETS_TESTS_TIMEOUT_FACTOR: 100
 
 install:

.travis.yml

@@ -2,7 +2,7 @@ env:
   global:
   # websockets only works on Python >= 3.4.
     - CIBW_SKIP="cp27-* cp33-*"
-    - CIBW_TEST_COMMAND="python3 -m unittest websockets"
+    - CIBW_TEST_COMMAND="python3 -W default -m unittest websockets"
     - WEBSOCKETS_TESTS_TIMEOUT_FACTOR=100
 
 matrix:

Makefile

@@ -1,12 +1,11 @@
 export PYTHONASYNCIODEBUG=1
-export PYTHONWARNINGS=default
 
 test:
-	python -m unittest
+	python -W default -m unittest
 
 coverage:
 	python -m coverage erase
-	python -m coverage run --branch --source=websockets -m unittest
+	python -W default -m coverage run --branch --source=websockets -m unittest
 	python -m coverage html
 
 clean:

tox.ini

@@ -12,7 +12,7 @@ commands =
     ; Before testing with speedups, compile the extension.
     speedups: python setup.py --quiet build_ext --inplace
 
-    python -m unittest {posargs}
+    python -W default -m unittest {posargs}
 
     ; After testing with speedups, remove the extension.
     speedups: sh -c 'rm websockets/*.so'
@@ -27,7 +27,7 @@ commands =
     python setup.py --quiet build_ext --inplace
 
     python -m coverage erase
-    python -m coverage run --branch --source=websockets -m unittest
+    python -W default -m coverage run --branch --source=websockets -m unittest
     python -m coverage report --show-missing --fail-under=100
 
     speedups: sh -c 'rm websockets/*.so'

websockets/protocol.py

@@ -15,6 +15,7 @@
 import logging
 import random
 import struct
+import warnings
 
 from .compatibility import asyncio_ensure_future
 from .exceptions import (
@@ -29,6 +30,16 @@
 logger = logging.getLogger(__name__)
 
 
+# On Python ≥ 3.7, silence a deprecation warning that we can't address before
+# dropping support for Python < 3.5.
+warnings.filterwarnings(
+    action='ignore',
+    message=r"'with \(yield from lock\)' is deprecated "
+            r"use 'async with lock' instead",
+    category=DeprecationWarning,
+)
+
+
 # A WebSocket connection goes through the following four states, in order:
 
 class State(enum.IntEnum):