Fix bug, check for dunder

[offlinemark]

allow sol funcs with 1 underscore, which is consistent with the above check in add_function

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This change is 

[disconnect3d]

For some reason travis hung :(
Can we also add a test for it?

[disconnect3d]

I restarted the build and it still hangs.

Can it be that we fall in some infinite recursion here?

[disconnect3d]

This check is wrong - the __private attributes are a syntax sugar for _CLS__private where CLS is the name of the class where a method that uses __private is defined.

In [9]: class A:
   ...:     def foo_invoker(self):
   ...:         print("A.foo_invoker")
   ...:         self.__foo()
   ...:
   ...:     def __foo(self):
   ...:         print("A.foo")
   ...:
   ...:     def __getattribute__(self, attr):
   ...:         print(f"Getting {attr}")
   ...:         return object.__getattribute__(self, attr)
   ...:

In [10]: A().foo_invoker()
Getting foo_invoker
A.foo_invoker
Getting _A__foo
A.foo

[disconnect3d]

On the other hand, this would still need to check for all protected attributes (the one starting with _) that the EVMAccount/EVMContract defines.

I've proposed a better fix for the issue we want to fix in #1306.