Collecting types from mocks causes a crash

[gvanrossum]

A user trying this in a test using mocks (in Python 2.7) reported crashes. She told me:

• pyannotate tries to get __mro__ from MagicMock when it gets constructed.
• MagicMock has 2 base classes — MagicMixin and Mock
• get_function_name_from_frame calls getattr(inst.__class__, '__mro__', None)
• Mock has __getattr__ method defined
• which tries to get _mock_methods:

      if name in ('_mock_methods', '_mock_unsafe'):
          raise AttributeError(name)
      elif self._mock_methods is not None:
          if name not in self._mock_methods or name in _all_magics:

• but _mock_methods has not been set yet
• because it gets set later in

  class MagicMixin(object):
  def __init__(self, *args, **kw):
      self._mock_set_magics()  # make magic work for kwargs in init
      _safe_super(MagicMixin, self).__init__(*args, **kw)
      self._mock_set_magics()  # fix magic broken by upper level init