depot_tools: Ignore python3 files generated by windows bootstrap.

Change-Id: I422c836258738ff11eb975361f56aa4ea7b4e2ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1764448
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

.gitignore

@@ -16,6 +16,8 @@
 /.pylint.d/
 /python
 /python.bat
+/python3
+/python3.bat
 /ssh.bat
 /ssh-keygen.bat
 /svn
@@ -33,6 +35,7 @@
 # Ignore locations where third-party tools are placed during bootstrapping.
 /python*_bin
 /python_bin_reldir.txt
+/python3_bin_reldir.txt
 /win_tools*_bin
 /git_bin
 /git-*_bin