Run gn_to_bp.py with python3

Python3 makes it quite difficult to have a single source file which can
be a module in a package and also runnable as a script [0]. The
gn_to_bp.py file is only a script and should not be considered in a gn
module, so call it as a script instead of as a module.

The gn_to_bp.py script can now take a path to the gn to use, so pass
that explicitly rather than implicitly on the PATH.

[0] https://stackoverflow.com/questions/16981921/relative-imports-in-python-3

Change-Id: I77da0c83033c2d402a348da5e1ee9741a33b4e6f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/842481
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed