Fix encoding for string comparisons on macOS

[INFO]      [exec] =============================== warnings summary ===============================
[INFO]      [exec] nuxeo-drive-client/tests/test_direct_edit.py::TestDirectEdit::test_filename_encoding
[INFO]      [exec]   /Users/jenkins/tmp/0ebd1d51/workspace/Drive_FT-drive_master-CUACR5UOIAO62W7K2CAAQ7UISS5UUL3SF5V2PSVOUZC4CRBQLRIA/sources/nuxeo-drive-client/nxdrive/direct_edit.py:542: UnicodeWarning: Unicode unequal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
[INFO]      [exec]     if name != file_name:
[INFO]      [exec] nuxeo-drive-client/tests/test_local_client.py::TestLocalClientNative::test_remote_changing_case_accentued_folder
[INFO]      [exec]   /Users/jenkins/tmp/0ebd1d51/workspace/Drive_FT-drive_master-CUACR5UOIAO62W7K2CAAQ7UISS5UUL3SF5V2PSVOUZC4CRBQLRIA/sources/nuxeo-drive-client/nxdrive/engine/watcher/local_watcher.py:822: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
[INFO]      [exec]     if (dst_path == normalize_event_filename(evt.src_path, action=False)
[INFO]      [exec] -- Docs: http://doc.pytest.org/en/latest/warnings.html
[INFO]      [exec] == 1 failed, 298 passed, 26 skipped, 2 xfailed, 2 warnings in 4356.46 seconds ==