Closes #8943:  COMPAT: periods needs coercion to np.int64
Let me know if this needs any work.

closes #8943 

does the test actually trigger the error?

pls add a release note (bug fixes in v0.16.0) as well
 Yes it does.  In fact, it was overkill, so I simplified the test and set `periods` to just 10.  I added some comments inside the test to explain what I'm testing for and why it overflows.

I tested it on 32-bit Linux (in VirtualBox) without the type conversions and it fails as expected.

```
======================================================================
FAIL: test_time_overflow_for_32bit_machines (pandas.tests.test_index.TestDatetimeIndex)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/garrettvb/Desktop/pandas/pandas/tests/test_index.py", line 1921, in test_time_overflow_for_32bit_machines
    self.assertEqual(len(idx1), periods)
AssertionError: 2 != 10
```
 Also, I added "(bug fixes in v0.16.0)" to the commit message.  Is that what you meant by "add a release note"?
 @Garrett-R see here for release notes: https://github.com/pydata/pandas/blob/master/doc/source/whatsnew/v0.16.0.txt

take a look at one of the older ones for examples.
 Oh, haha, thanks!  I'll take a look.
 Sorry about the typo.  OK, hopefully everything's in order now.
 Thanks @Garrett-R !
