Reland "[infra] Use Python3 for our Presubmits"

This is a reland of commit 094bcdb9e5dcb2ebe0ef780faf0559ab72b50bac

Original change's description:
> [infra] Use Python3 for our Presubmits
>
> https://source.chromium.org/chromium/chromium/tools/depot_tools/+/main:presubmit_support.py;l=319;drc=443d9135cc33f3156d5fe25ebec33f9adffbab65
>
> This also makes any errors from `make train -C infra/bots`
> look well formatted because check_output returns a bytestring
> in Python3 and when that is printed, the newlines et al are not
> rendered correctly. Thus we want the output of check_output
> to be encoded to UTF-8.
>
> Without setting the USE_PYTHON3 = True in PRESUBMIT.py,
> it appears that `git cl upload` would try to run our
> infra_tests.py in Python2 mode, which does not have
> the encoding argument for check_output.
>
> Apparently cipd_bin_packages/vpython3 does not have the
> "six" package installed (but cipd_bin_packages/vpython does)
> so I replaced the six.StringIO with io.StringIO, which
> is where that is located in Python3.
>
> Change-Id: Ic8f61bf943531583ba3d110a85260d69bdbf5eb2
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/537677
> Reviewed-by: Ravi Mistry <rmistry@google.com>
> Reviewed-by: Eric Boren <borenet@google.com>

Change-Id: Ia7fb2f3b6d8b70ca95cf10763782d4a0122053e0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/537978
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
6 files changed