check-package: fix Python3 support

check-package: fix Python3 support

This script currently uses "/usr/bin/env python" as shebang but it does
not really support Python3. Instead of limiting the script to Python2,
fix it to support both versions.

So change all imports to absolute imports because Python3 follows PEP328
and dropped implicit relative imports.

In order to avoid errors when decoding files with the default 'utf-8'
codec, use errors="surrogateescape" when opening files, the docs for
open() states: "This is useful for processing files in an unknown
encoding.". This argument is not compatible with Python2 open() so
import 'six' to use it only when running in Python3.
As a consequence the file handler becomes explicit, so use it to close()
the file after it got processed.

This "surrogateescape" is a simple alternative to the complete solution
of opening files with "rb" and changing all functions in the lib*.py
files to use bytes objects instead of strings. The only case we can have
non-ascii/non-utf-8 files being checked by the script are for patch
files when the upstream file to be patched is not ascii or utf-8. There
is currently one case in the tree:
package/urg/0002-urg-gcc6-fix-narrowing-conversion.patch.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Tested-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>