Update PRESUBMIT.py

Updates PRESUBMIT.py for pdfium_test for Python 3, and presubmit 2.0.

Also enables pan-project checks, and fix the existing violation.

Bug: pdfium:1962
Change-Id: I9e97e8d26a3a9f72e771f616989fc61ee422b127
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/102870
Reviewed-by: Nigi <nigi@chromium.org>