CLN: Remove py3 next method from FixedWidthReader

It's unnecessary - handle wraps this instead.
Merge pull request #4923 from jtratner/simplify-next-in-fwf

CLN: Remove py3 next method from FixedWidthReader
BUG: Fix bound checking for Timestamp() with dt64 #4065

To fix the bug, this change adds bounds checking to
_get_datetime64_nanos() for numpy datetimes that aren't already in [ns]
units.

Additionally, it updates _check_dts_bounds() to do the bound check just
based off the pandas_datetimestruct, by comparing to the minimum and
maximum valid pandas_datetimestructs for datetime64[ns].  It is simpler
and more accurate than the previous system.