i18n_tool.py: fix Unicode handling; change commit format

[rmol]

Status

Ready for review

Description of Changes

Use of six.u was corrupting translator names. According to
https://pythonhosted.org/six/index.html#six.u -- "On Python 2, u()
doesn't know what the encoding of the literal is. Each byte is
converted directly to the unicode codepoint of the same value. Because
of this, it's only safe to use u() with strings of ASCII data."

Of course some translators' names were not just ASCII, so I've
replaced usage of six.u with u"", as it should be safe on both Python
2 and Python 3 > 3.3, per PEP-0414.

I also changed the commit format to put each contributor's name on its
own line, and tried to clarify the source of the translations. It was
confusing to me, at least, that translations were being "copied from"
a revision that involved a different language.

Finally, we were gathering the list of contributors from commits other
than just the Weblate contributions, so whoever last ran the tool was
getting credited for translations since. I've changed the tool to only
look for contributors in the automated Weblate commits.

Testing

1. Check out this branch.
2. Create a testing branch from it with git checkout -b i18npy2 i18n_tool.
3. Run i18n_tool.py under Python 2:
   PYTHON_VERSION=2 securedrop/bin/dev-shell python2 ./i18n_tool.py --verbose update-from-weblate
4. Check the commits added to your i18npy2 branch. There should be a commit for each supported locale, added in alphabetic order, with only a few contributors listed for each. The names of contributors to Swedish, Norwegian, and Icelandic translations should appear correctly, with no line noise.
5. Create a testing branch from it with git checkout -b i18npy3 i18n_tool.
6. Run i18n_tool.py under Python 3:
   PYTHON_VERSION=3 securedrop/bin/dev-shell python3 ./i18n_tool.py --verbose update-from-weblate
7. Check the commits added to your i18npy3 branch.

Deployment

These changes should only affect development.

Checklist

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR