The `utctz_to_altz` and `altz_to_utctz_str` functions fail to handle
timezones with UTC offsets that are not a multiple of one
hour. Rewrite them and add some unit tests.

Fixes #630

GitPython parses the output of `git diff --numstat` to get the
files changed in a commit.
This breaks when a commit contains a file rename, because the output
of `git diff` is different than expected.
This is the output of a normal commit:

    $ git diff --numstat 8f41a390bf9a^ 8f41a390bf9a
    30      5       test/test_repo.py

And this a commit containing a rename:

    $ git diff --numstat 185d847ec764^ 185d847ec764
    3       1       .github/workflows/{test_pytest.yml => Future.yml}

This can be triggered by this code:

    for commit in repo.iter_commits():
        print(commit.hexsha)
            for file in commit.stats.files:
                print(file)

Which will print for the normal commit:

    8f41a390bf9a54db6f85032bc56b453307b95451
    'test/test_repo.py'

And when there is a rename:

    185d847ec7647fd2642a82d9205fb3d07ea71715
    '.github/workflows/{test_pytest.yml => Future.yml}'

Additionally, when a path member is removed, the file list become
a list of strings, breaking even more the caller. This is in the
Linux kernel tree:

    $ git diff --numstat db401875f438^ db401875f438
    1       1       tools/testing/selftests/drivers/net/mlxsw/{spectrum-2 => }/devlink_trap_tunnel_ipip6.sh

and GitPython parses it as:

    db401875f438168c5804b295b93a28c7730bb57a
    ('tools/testing/selftests/drivers/net/mlxsw/{spectrum-2 => '
    '}/devlink_trap_tunnel_ipip6.sh')

Fix this by pasing the --no-renames option to `git diff` which ignores
renames and print the same output as if the file was deleted from the
old path and created in the new one:

    $ git diff --numstat --no-renames 185d847ec764^ 185d847ec764
    57      0       .github/workflows/Future.yml
    0       55      .github/workflows/test_pytest.yml

These errors are mostly fixed by either adding blank lines or single
spaces for Sphinx documentation key words.

The commit solely includes documentation changes, no functional
changes.