Change default timezone to UTC
Commit 107ce3e3721c5517f345595a5740f2108e40e015 changed the pytz

requirement to be >= 2012h because versions earlier than that do

not support the US/Pacific designation of the timezone, which

Review Board was using as a default. This makes it difficult or

impossible to install Review Board on Linux distributions with an

older version of pytz.
This patch changes the default timezone to UTC, which should be

available in all versions of pytz since time out of memory. With

this, we can drop the pytz strict version requirement as well,

trusting that the platform version will support UTC.