mangling of double-underscore method argument names

[robertwb]

CPython apparently does this:

class Spam:
    def eggs(__some_kwarg:int=None):
        print(__some_kwarg)

>>> Spam.eggs(10)
10
>>> Spam.eggs(_Spam__some_kwarg=10)
10

class Foo:
  def bar(self, *, __kw:'anno'='default'):
    pass

>>> Foo.bar.__annotations__
{'_Foo__kw': 'anno'}
>>> Foo.bar.__kwdefaults__
{'_Foo__kw': 'default'}

http://bugs.python.org/issue20625

Migrated from http://trac.cython.org/ticket/826

[scoder]

Nominating as release blocker since it's a bit of a breaking change (although not very like to occur in practice).