Fix to _round for timestamps to accommodate numpy int64

Also minor PEP8 edits to timestamp tests

Author: cbertinato

pandas/_libs/tslibs/timestamps.pyx

@@ -601,7 +601,6 @@ class Timestamp(_Timestamp):
             value = self.tz_localize(None).value
         else:
             value = self.value
-
         if unit < 1000:
             # for nano rounding, work with the last 6 digits separately
             # due to float precision
@@ -617,9 +616,9 @@ class Timestamp(_Timestamp):
             if unit >= 1e9:
                 divisor = 10 ** int(np.log10(unit / 1e7))
             else:
-                divisor = 1
+                divisor = 10
 
-            r = (unit * rounder((value * divisor / float(unit)) / divisor)
+            r = (unit * rounder((value * (divisor / float(unit))) / divisor)
                  .astype('i8'))
 
         result = Timestamp(r, unit='ns')

pandas/tests/scalar/test_timestamp.py

@@ -13,7 +13,6 @@
 from datetime import datetime, timedelta
 from distutils.version import LooseVersion
 from pytz.exceptions import AmbiguousTimeError, NonExistentTimeError
-from collections import namedtuple
 
 import pandas.util.testing as tm
 import pandas.util._test_decorators as td
@@ -749,11 +748,15 @@ def test_to_pydatetime_nonzero_nano(self):
         ('20130101 09:10:11', 'floor', 'D', '20130101', {}),
         # GH 19206 - times far in the future and past rounding incorrectly
         ('2117-01-01 00:00:45', 'floor', '15s', '2117-01-01 00:00:45', {}),
+        ('2117-01-01 00:00:45', 'ceil', '15s', '2117-01-01 00:00:45', {}),
         ('2117-01-01 00:00:45.000000012', 'floor', '10ns',
          '2117-01-01 00:00:45.000000010', {}),
         ('1823-01-01 00:00:01', 'floor', '1s', '1823-01-01 00:00:01', {}),
+        ('1823-01-01 00:00:01', 'ceil', '1s', '1823-01-01 00:00:01', {}),
         ('1823-01-01 00:00:01.000000012', 'floor', '10ns',
          '1823-01-01 00:00:01.000000010', {}),
+        ('1823-01-01 00:00:01.000000012', 'ceil', '10ns',
+         '1823-01-01 00:00:01.000000020', {}),
         # ----
         ('20130101 09:10:11', 'ceil', 'D', '20130102', {}),
         ('20130101 09:10:11', 'round', 'D', '20130101', {}),