Change python to python3

the problem is that sometimes /usr/bin/python does not exist.
For example: debian 11 has been confirmed that the python command will not be provided.

Change-Id: I9bbe08ed10b126b92e9a43613fc379a84e614422
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/11120
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
diff --git a/src/gn/exec_process_unittest.cc b/src/gn/exec_process_unittest.cc
index 5ca0605..599892d 100644
--- a/src/gn/exec_process_unittest.cc
+++ b/src/gn/exec_process_unittest.cc
@@ -33,7 +33,7 @@
   args.push_back(L"-c");
   args.push_back(base::UTF8ToUTF16(command));
 #else
-  args.push_back("python");
+  args.push_back("python3");
   args.push_back("-c");
   args.push_back(command);
 #endif