Fix compatibility with pytest 3.8.1

Apparently for a test *method*, item.obj and item.function are
different: item.obj is the bound method, and item.function is the
underlying unbound method. At least in pytest 3.8.1.

We want to wrap the bound method, not the unbound method, so we should
wrap item.obj, not item.function.

Author: njsmith

newsfragments/64.bugfix.rst

@@ -0,0 +1,2 @@
+The pytest 3.8.1 release broke pytest-trio's handling of trio tests
+defined as class methods. We fixed it again.

pytest_trio/plugin.py

@@ -292,7 +292,7 @@ async def run(self, test_ctx, contextvars_ctx):
 
 
 def _trio_test_runner_factory(item, testfunc=None):
-    testfunc = testfunc or item.function
+    testfunc = testfunc or item.obj
 
     if getattr(testfunc, '_trio_test_runner_wrapped', False):
         # We have already wrapped this, perhaps because we combined Hypothesis