Set script_executable to "python3"

Sets script_executable to "python3" in the .gn file. This changes the
executable for exec_script rules, which currently assume that "python"
is a Python 3 interpreter; "python" typically is a Python 2 interpreter.

Explanatory comment taken from Chromium.

R=thestig@chromium.org

Bug: pdfium:1674
Change-Id: Ie758424da20af8f6f482596a781b854dfd4365c3
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/85650
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: K. Moon <kmoon@chromium.org>
diff --git a/.gn b/.gn
index 836045e..32c0131 100644
--- a/.gn
+++ b/.gn
@@ -4,6 +4,10 @@
 
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
+# The python interpreter to use by default. On Windows, this will look
+# for python3.exe and python3.bat.
+script_executable = "python3"
+
 default_args = {
   v8_extra_library_files = []
   v8_experimental_extra_library_files = []