cross filesystems

Added

• pyproject.toml to specify build dependencies & build backend

Removed

• compatibility with python3.5 & python3.6

Fixed

• fix export when output and temporary directory are on different filesystems (#85)

restore python3.5 compatibility

Fixed

• python3.5:
  • TypeError in _write_event when renaming temporary file
  • TypeError in _sync_event when reading file
  • tests: TypeError when converting to pathlib.Path
  • tests: AttributeError due to unavailable MagicMock.assert_called_once