bpo-34166: Use tobytes instead of tostring and use with statement for file handling

[tirkarthi]

Changes made :

• Use tobytes instead of tostring.
• Use with statement for file handling to fix ResourceWarning.

Tested using a sample .po file as in https://bugs.python.org/msg322074

I think this needs a backport to 3.7 and 3.6. I haven't added a NEWS entry since the changes seem trivial let me know if it's needed.

Thanks

https://bugs.python.org/issue34166

[miss-islington]

Thanks @tirkarthi for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6, 3.7.
🐍🍒⛏🤖

[miss-islington]

Sorry @tirkarthi and @serhiy-storchaka, I had trouble checking out the 3.7 backport branch.
Please backport using cherry_picker on command line.
cherry_picker a692efe4733f98831cb51a9683877b152f754d14 3.7

[bedevere-bot]

GH-8368 is a backport of this pull request to the 3.6 branch.

[bedevere-bot]

GH-8369 is a backport of this pull request to the 3.7 branch.

[tirkarthi]

Strangely enough there weren't any conflicts for 3.7 branch when I used cherry_picker command but still @miss-islington failed to make a PR.

➜  cpython git:(master) ✗ cherry_picker a692efe4733f98831cb51a9683877b152f754d14 3.7
🐍 🍒 ⛏
From github.com:python/cpython
   06ca3f0..a692efe  master     -> upstream/master

Now backporting 'a692efe4733f98831cb51a9683877b152f754d14' into '3.7'
Switched to a new branch 'backport-a692efe-3.7'
Branch backport-a692efe-3.7 set up to track remote branch 3.7 from upstream.

[backport-a692efe-3.7 00faa43] bpo-34166: Fix warnings in Tools/msgfmt.py. (GH-8367)
 Author: Xtreak <tirkarthi@users.noreply.github.com>
 Date: Sat Jul 21 11:52:12 2018 +0530
 1 file changed, 5 insertions(+), 3 deletions(-)

To git@github.com:tirkarthi/cpython.git
 * [new branch]      backport-a692efe-3.7 -> backport-a692efe-3.7

Backport PR URL:
https://github.com/python/cpython/compare/3.7...tirkarthi:backport-a692efe-3.7?expand=1
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.

Deleted branch backport-a692efe-3.7 (was 137aade).

branch backport-a692efe-3.7 has been deleted.