Add Annotations to assert_*_equal functions

[WillAyd]

Follow up to #25462 (comment) want to add annotations for parameters.

Would ideally probably want to add NoReturn but that requires 3.5.4 at a minimum so may want to explore other options

[samuelsinayoko]

Am going to try and pick this one up. Am interested to help with mypy and I have worked on assert utils as part of #25462 (comment).

[avinashpancham]

made an overview of the functions in _testing/asserters.py. I will start with assert_frame_equal and assert_series_equal. Issue seems like a good first issue

• assert_almost_equal
• assert_dict_equal
• assert_index_equal
• assert_class_equal
• assert_attr_equal
• assert_categorical_equal
• assert_interval_array_equal
• assert_period_array_equal
• assert_datetime_array_equal
• assert_timedelta_array_equal
• assert_numpy_array_equal
• assert_extension_array_equal
• assert_series_equal
• assert_frame_equal
• assert_equal
• assert_sp_array_equal

[mroeschke]

I see a lot of these methods are typed now so closing