Add shebang to servlet.py

Per go/cros-python3-howto, this is the shebang that should be kept
compatible with both Python2 and Python3. Autotest is fragile, so I'm
playing it safe here.

BUG=chromium:1031760
TEST=Run servlet.py in both python2 and python3, and run lib2to3 on
servlet.py

Change-Id: Ia5125bbd075defe69e20523faf36ea58be784c2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/1959080
Tested-by: Greg Edelston <gredelston@google.com>
Reviewed-by: Max Timkovich <timkovich@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Greg Edelston <gredelston@google.com>
diff --git a/provingground/av_analysis/recording_server/servlet.py b/provingground/av_analysis/recording_server/servlet.py
index e0f812f..ea9c31b 100644
--- a/provingground/av_analysis/recording_server/servlet.py
+++ b/provingground/av_analysis/recording_server/servlet.py
@@ -1,3 +1,4 @@
+#!/usr/bin/env python
 import os
 import subprocess