Follow Ronny's advice and use `type in base_type`.

_pytest/python_api.py

@@ -2,6 +2,7 @@ import math
 import sys
 
 import py
+from six import binary_type, text_type
 from six.moves import zip, filterfalse
 from more_itertools.more import always_iterable
 
@@ -584,11 +585,11 @@ def raises(expected_exception, *args, **kwargs):
 
     """
     __tracebackhide__ = True
-    if not isclass(expected_exception) or not issubclass(expected_exception, BaseException):
-        for exc in filterfalse(isclass, always_iterable(expected_exception)):
-            msg = ("exceptions must be old-style classes or"
-                   " derived from BaseException, not %s")
-            raise TypeError(msg % type(exc))
+    base_type = (type, text_type, binary_type)
+    for exc in filterfalse(isclass, always_iterable(expected_exception, base_type)):
+        msg = ("exceptions must be old-style classes or"
+                " derived from BaseException, not %s")
+        raise TypeError(msg % type(exc))
 
     message = "DID NOT RAISE {0}".format(expected_exception)
     match_expr = None