Support subclassing StrictMock

[fornellas]

This should work:

from testslide import StrictMock

class Template:
  def __eq__(self, other):
    return False

class TemplateStrictMock(StrictMock):
  def __init__(self):
    super().__init__(template=Template)

  def __eq__(self, other):
    return False

mock = TemplateStrictMock()
assert (mock == mock) is False

But is currently failing with:

testslide.strict_mock.UndefinedAttribute: '__eq__' is not set.
<StrictMock 0x7F6A9E5F4D90 template=__main__.Template test.py:9> must have a value set for this attribute if it is going to be accessed.