[infra] Fix dangling Python2-specific issues

subprocess.check_output needs encoding or it defaults
to bytes.

Python 3 has / be float division and // be integer
division.

The skpbench fix is speculative.

Change-Id: I13d0e976c03bba30761c8ab533e6fdd041b795c8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/541696
Reviewed-by: Ravi Mistry <rmistry@google.com>
11 files changed