Revert "Make test_env.py compatible w/ Python 3"

This reverts commit 1102f12d8e3317d18baae355789e893aba57a325.

Reason for revert: it is breaking stuff on several Mac bots.

[E2019-09-12T12:28:01.848585-07:00 31162 0 annotate.go:241] goroutine 1:
[E2019-09-12T12:28:01.848653-07:00 31162 0 annotate.go:241] #0 go.chromium.org/luci/vpython/python/find.go:93 - python.Find()
[E2019-09-12T12:28:01.848672-07:00 31162 0 annotate.go:241]   annotation #0:
[E2019-09-12T12:28:01.848683-07:00 31162 0 annotate.go:241]     reason: no Python found
[E2019-09-12T12:28:01.848692-07:00 31162 0 annotate.go:241]   annotation #1:
[E2019-09-12T12:28:01.848701-07:00 31162 0 annotate.go:241]     reason: could not find appropriate executable for: "python3.8"
[E2019-09-12T12:28:01.848709-07:00 31162 0 annotate.go:241]   annotation #2:
[E2019-09-12T12:28:01.848725-07:00 31162 0 annotate.go:241]     reason: could not find target in system
[E2019-09-12T12:28:01.848739-07:00 31162 0 annotate.go:241]     internal reason: target(python3.8)/dirs([/b/s/w/ir/.swarming_module /b/s/w/ir/.swarming_module/cpython/bin /b/s/w/ir/.swarming_module/cpython3/bin /b/s/cipd_cache/bin /opt/local/bin /opt/local/sbin /usr/local/sbin /usr/local/git/bin /usr/local/bin /usr/sbin /usr/bin /sbin /bin])
[E2019-09-12T12:28:01.848764-07:00 31162 0 annotate.go:241] 
[E2019-09-12T12:28:01.848774-07:00 31162 0 annotate.go:241] #1 go.chromium.org/luci/vpython/venv/config.go:272 - venv.(*Config).resolvePythonInterpreter()
[E2019-09-12T12:28:01.848785-07:00 31162 0 annotate.go:241]   reason: could not find Python for: 3.8.0
[E2019-09-12T12:28:01.848799-07:00 31162 0 annotate.go:241] 
[E2019-09-12T12:28:01.848808-07:00 31162 0 annotate.go:241] #2 go.chromium.org/luci/vpython/venv/config.go:187 - venv.(*Config).makeEnv()
[E2019-09-12T12:28:01.848822-07:00 31162 0 annotate.go:241]   reason: failed to resolve system Python interpreter
Original change's description:
> Make test_env.py compatible w/ Python 3
> 
> Makes test_env.py compatible with both Python 2 and 3. Also adds a very
> simple test to ensure that it remains compatible until some other test
> switches to Python 3.
> 
> As a side effect of adding the test, also makes
> testing/scripts/common.py compatible with Python 3 since it's
> necessary for running isolated scripts.
> 
> Bug: 898348
> Change-Id: Ib58cf4c4f85df60a432764d7f744bc3fee09ae3a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796525
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#695825}

TBR=dpranke@chromium.org,bsheedy@chromium.org

Change-Id: I80fb04b1a224db48b53865e871f39b6364a785b0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 898348
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1802095
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696156}