This is a reland of 5bfa3ae8

Replace cStringIO with StringIO and add tests.

Original change's description:
> gerrit_util: Refactor ReadHttpResponse and add more tests.
>
> Bug: 1016601
> Change-Id: Ie6afc5b1ea29888b0bf40bdb39b2b492d2d0494c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1880014
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1016601
Change-Id: I0c83a83202169b6a1acc60bdf6f4cd00eac6e2a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1884461Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

This reverts commit 5bfa3ae8.

Reason for revert:
Fails when fetching status for cl 1708084 

Original change's description:
> gerrit_util: Refactor ReadHttpResponse and add more tests.
> 
> Bug: 1016601
> Change-Id: Ie6afc5b1ea29888b0bf40bdb39b2b492d2d0494c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1880014
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,apolito@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1016601
Change-Id: I84bc378ed5f58e911e0900b1a5dbea70dc06ade1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1883677Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1016601
Change-Id: Ie6afc5b1ea29888b0bf40bdb39b2b492d2d0494c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1880014Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

This reverts commit a834f39e.

Reason for revert:
https://bugs.chromium.org/p/chromium/issues/detail?id=1017367

Original change's description:
> presubmit_support: Fix tests on Python 3.
>
> Bug: 1009814
> Change-Id: I2c2a835fc4fa0a7fc9e68dfff680fb0beacb491e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1869535
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,apolito@google.com

Bug: 1009814
Change-Id: I48907caeca9b9b32e4e3a7cb5a636280d017a4c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1877350Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1009814
Change-Id: I2c2a835fc4fa0a7fc9e68dfff680fb0beacb491e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1869535Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Newer builds of vpython contain vpython3 as well.

Bug: 898348
Change-Id: I63b197690e2ae47bd4c6cd50894c4769e1a18334
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1727581Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: smut <smut@google.com>