ENH: Add dateutil timezone support GH4688

Adds support for dateutil timezones across pandas.
Support conversion between dateutil and pytz timezones.
Ensure timezone functionality is tested with both dateutil and pytz.
Merge pull request #6968 from ahlmss/AHLMSS_0.13.1_ahl1

ENH: Support dateutil timezones. GH4688.
UNI/HTML: add encoding argument to read_html
Merge pull request #7323 from cpcloud/html-encoding

UNI/HTML/WIP: add encoding argument to read_html