[3.5] bpo-30822: Fix testing of datetime module. (GH-2530) (GH-2783)

[musically-ut]

Only C implementation was tested..
(cherry picked from commit 287c559)

https://bugs.python.org/issue30822

[larryhastings]

Just a reminder: I expect to tag 3.5.4rc1 in eleven or twelve hours. If this is a bugfix (not a security fix), and it isn't merged by then, it will probably never be merged into 3.5.

[musically-ut]

This is a relatively minor bugfix (just fixes an export _divide_and_round which shouldn't have happened and re-enables some tests which are not visible to the end-user anyway).

So not much to be gained but maybe something to be lost (there is a faint possibility of some code out there using that removed export).

[vstinner]

@larryhastings: "Just a reminder: I expect to tag 3.5.4rc1 in eleven or twelve hours. If this is a bugfix (not a security fix), and it isn't merged by then, it will probably never be merged into 3.5."

Sorry @musically-ut, we missed the deadline. The problem is not enabling tests. If tests start to fail, it will be harder to justify to apply bugfixes in the 3.5 branch. Let's focus on 2.7, 3.6 and master branches ;-)