Fix ISO date and time format (#66)

The separator for milliseconds is a dot according to ISO 8601.

Author: penguinpee

tests/datetime/test_breaks.py

@@ -10,7 +10,7 @@
 
 
 def test_by_n_microseconds():
-    l1 = dt(("2025-01-01 01:10:30:000250", "2025-01-01 01:10:30:000600"))
+    l1 = dt(("2025-01-01 01:10:30.000250", "2025-01-01 01:10:30.000600"))
     assert by_n(l1) == [
         datetime(2025, 1, 1, 1, 10, 30, 200),
         datetime(2025, 1, 1, 1, 10, 30, 300),
@@ -264,7 +264,7 @@ def test_by_n_century():
 
 
 def test_by_width_microseconds():
-    l1 = dt(("2025-01-01 01:10:30:000250", "2025-01-01 01:10:30:000600"))
+    l1 = dt(("2025-01-01 01:10:30.000250", "2025-01-01 01:10:30.000600"))
     assert by_width(l1, "250 microseconds") == [
         datetime(2025, 1, 1, 1, 10, 30, 250),
         datetime(2025, 1, 1, 1, 10, 30, 500),