tests: target DateTime 3 and use mock to monkey patch

mock has several advantages, the main one here is that it errors when
patching does not replace an existing attribute, which happens when we
are not patching the right place.

These attributes are not the same place in DateTime 2 and 3, this code
when using DateTime 2 was replacing the attributes, but in DateTime 3 it
was just creating new attributes that were never used. Update the code
to patch the DateTime 3 location