nebraska: Prepare to migrate to chromite.

Fix issues raised in:
https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1906719/6#message-4d3f4795212d9ed5e44fb0213087e755816f1c55

* proper python3 bytes<->text conversion
* import datetime
* Replace NebraskaError with Error
* 2 newlines between top level classes/functions
* Remove NebraskaUnittest
* Use assertIn, assertEqual, assertIsNotNone instead of
assertTrue/assertFalse.

BUG=chromium:1003986
TEST=nebraska_unittest, python3 nebraska_unittest.py,
devserver_integration_test, cros flash.

Change-Id: I6edbc300fd5f58c4456c5a9f86f71af931a995f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1910343
Tested-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
2 files changed