Reapply "debug! loop fixture"

This reverts commit f35189b.

tests/test_proxy_functional.py

@@ -98,6 +98,7 @@ async def handler(*args, **kwargs):
 # Filter out the warning from
 # https://github.com/abhinavsingh/proxy.py/blob/30574fd0414005dfa8792a6e797023e862bdcf43/proxy/common/utils.py#L226
 # otherwise this test will fail because the proxy will die with an error.
+@pytest.mark.usefixtures("loop")
 async def test_secure_https_proxy_absolute_path(
     client_ssl_ctx: ssl.SSLContext,
     secure_proxy_url: URL,