Only use @pytest.fixture decorator once per fixture

This is now required in pytest >= 3.6

We define our own autoused fixtue not to change the API

See pytest-dev/pytest#3518

Fixes kevin1024#49

Author: hroncok

tests/conftest.py

@@ -3,4 +3,6 @@
 from pytest_httpbin.plugin import httpbin_ca_bundle
 
 
-pytest.fixture(autouse=True)(httpbin_ca_bundle)
+@pytest.fixture(autouse=True, scope='function')
+def httpbin_ca_bundle_autoused(httpbin_ca_bundle):
+    pass