This reverts commit eb16430c.

Reason for revert: blocking upload of CLs, https://crbug.com/1357152#c8

Original change's description:
> Use io.open for opening files
>
> Use io.open which is consistent in py2 and py3.
>
> Bug: 1357152
> Change-Id: I49a3df503026bc6918362a9b5600f57111111111
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3872429
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Bug: 1357152
Change-Id: Iec18fe8fd37f28887507d47d7b35b93ccee72375
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3872487
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Gavin Mak <gavinmak@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>