make sure to run async tests

there are some `async def` tests, but they are skipped without `mark("asyncio")`

IPython/conftest.py

@@ -1,10 +1,12 b''
-import types
-import sys
 import builtins
+import inspect
 import os
-import pytest
 import pathlib
 import shutil
+import sys
+import types
+
+import pytest
 
 # Must register before it gets imported
 pytest.register_assert_rewrite("IPython.testing.tools")
@@ -12,6 +14,19 b' pytest.register_assert_rewrite("IPython.testing.tools")'
 from .testing import tools
 
 
+def pytest_collection_modifyitems(items):
+    """This function is automatically run by pytest passing all collected test
+    functions.
+
+    We use it to add asyncio marker to all async tests and assert we don't use
+    test functions that are async generators which wouldn't make sense.
+    """
+    for item in items:
+        if inspect.iscoroutinefunction(item.obj):
+            item.add_marker("asyncio")
+        assert not inspect.isasyncgenfunction(item.obj)
+
+
 def get_ipython():
     from .terminal.interactiveshell import TerminalInteractiveShell
     if TerminalInteractiveShell._instance:

setup.py

@@ -150,6 +150,7 b' extras_require = dict('
     doc=["Sphinx>=1.3"],
     test=[
         "pytest",
+        "pytest-asyncio",
         "testpath",
         "pygments",
     ],