luci-py: Making __file__ usage unicode safe.

In luci-py, all paths should be unicode.

__file__ is a str object in the file system's encoding, hence you need to use
`__file__.decode(sys.getfilesystemencoding())` to convert it into unicode.

Fixes #264.

Review-Url: https://codereview.chromium.org/2093593002
NOKEYCHECK=True
GitOrigin-RevId: 35ec08d6007a823bf39a5095c8bc98a6865b8efd