More Python3 deparsing

- grammar rule genexpr
- More Python3 docstring formatted

test/simple_source/comprehension/10-genexpr.py

@@ -0,0 +1,7 @@
+# Tests in Python 3:
+#
+# load_genexpr ::= BUILD_TUPLE_1 LOAD_GENEXPR LOAD_CONST
+# genexpr ::= load_closure load_genexpr MAKE_CLOSURE_0 expr GET_ITER CALL_FUNCTION_1
+
+def __instancecheck__(cls, instance):
+    return any(cls.__subclasscheck__(c) for c in {subclass, subtype})