Consider start support for #= for new workflows in .po format

[ikks]

Hi, according to gettext project, there is a new addition to the format that involves lines starting with #=.

At the moment polib raises an exception when it finds a line starting with #= as in

File "python3.13/site-packages/polib.py", line 130, in pofile
    return _pofile_or_mofile(pofile, 'pofile', **kwargs)
  File "python3.13/site-packages/polib.py", line 78, in _pofile_or_mofile
    instance = parser.parse()
  File "python3.13/site-packages/polib.py", line 1456, in parse
    raise IOError('Syntax error in po file %s(line %s)' %
                  (fpath, self.current_line))
OSError: Syntax error in po file apt_es.po (line 116)

As starting point it would be a reasonable approach just to be able to read and store the new format lines as they are present in the source file.

Optionally, there could be a method workflow_entries() to list the entries that have #= present.