btrfs/154: migrate to python3

btrfs/154: migrate to python3

Test case btrfs/154 is still using python2 script, which is already EOL.
Some rolling distros like Archlinux is no longer providing python2
package anymore.

This means btrfs/154 will be harder and harder to run.

To fix the problem, migreate the python script to python3, this involves
the following changes:

- Change common/config to use python3
- Strong type conversion between string and bytes
  This means, anything involved in the forged bytes has to be bytes.

  And there is no safe way to convert forged bytes into string, unlike
  python2.
  I guess that's why the author went python2 in the first place.

  Thankfully os.rename() still accepts forged bytes.

- Use bytes specific checks for invalid chars.

The updated script can still cause the needed conflicts, can be verified
through "btrfs ins dump-tree" command.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>