Switch deqp merge to python3

Got broken by https://crrev.com/c/3600911 and that somehow doesn't
break builds, just says "json.output (invalid)" as the script raised
an exception (ImportError: No module named pathlib).

The script is executed using sys.executable so changing shebang doesn't
work.

Bug: angleproject:6854
Change-Id: I38d72755ed9d1d0983439e017c616cfbe8c52b17
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3634727
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

scripts/angle_deqp_test_merge.py

@@ -6,9 +6,14 @@
 """ Merges dEQP sharded test results in the ANGLE testing infrastucture."""
 
 import os
-import pathlib
 import sys
+if sys.version_info.major != 3 and __name__ == '__main__':
+    # Swarming prepends sys.executable so we get python2 regardless of shebang.
+    # Spawn itself with vpython3 instead.
+    import subprocess
+    sys.exit(subprocess.call(['vpython3', os.path.realpath(__file__)] + sys.argv[1:]))
 
+import pathlib  # python3
 
 PY_UTILS = str(pathlib.Path(__file__).resolve().parents[1] / 'src' / 'tests' / 'py_utils')
 if PY_UTILS not in sys.path: