Fix period raising exceptions in __eq__()

[gnattishness]

Period can raise an AttributeError when being compared with other types.

e.g.

>>> import pendulum
>>> p = pendulum.now() - pendulum.yesterday()
>>> p == None
AttributeError: 'NoneType' object has no attribute 'start'
>>> # should return False

This fix retains the same duck-typing capabilities, but using isinstance() is another option.