Convert contrib/mklog script to Python 3

Convert contrib/mklog script to Python 3

Upstream will drop support for Python 2.x on January 1, 2020.  This
patch converts the contrib/mklog script to Python 3.  The process for
the conversion was as follows.

- Use the futurize tool (https://python-future.org ) to apply the
  print_with_import, except, and dict transformations.

- Remove the "from __future__ import print_function".

- Change the shebang line to search for python3 in the environment.

- Modify the run() function to return a str instead of bytes.

- Update the copyright year.

contrib/ChangeLog:

2019-05-21  Janne Blomqvist  <jb@gcc.gnu.org>

        * mklog: Convert to Python 3.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271456 138bc75d-0d04-0410-961f-82ee72b054a4