Inihbit AST transformations inside ClassDef nodes

Without this inhibition, the following test case

    >>> %reload_ext awaitless
    >>> import asyncio
    >>> class Foo:
    ...     x = asyncio.sleep(0)

would trigger AST transformation - and we can't use awaits in class
definitions.

awaitless/__init__.py

@@ -75,6 +75,17 @@ def visit_AsyncFunctionDef(self, node):
 
         return node
 
+    def visit_ClassDef(self, node):
+        """
+        Inhibit AST transformation inside ClassDef nodes.
+
+        We need to prevent assignments inside class bodies from being
+        transformed, since an "await" there is illegal. (Functions defined in
+        class bodies are already covered.)
+        """
+
+        return node
+
     def visit_Expr(self, node):
         """
         Rewrite Expr nodes

test.py

@@ -122,3 +122,11 @@ def doctest_lambda():
     The actual conversion from coroutine to task occurred in the last line,
     which was an Expr.
     """
+
+
+def doctest_assignment_in_classdef():
+    """
+    >>> %reload_ext awaitless
+    >>> import asyncio
+    >>> class Foo: x = asyncio.sleep(0)
+    """