Extracting messages from py files with f-strings.

[atom32k]

Hi,

Looks like python ast module literal_eval doesn't play well with f-strings in code.

Traceback (most recent call last):
  File "/home/vagrant/venv/bin/pot-create", line 8, in <module>
    sys.exit(main())
  File "/home/vagrant/venv/lib/python3.8/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/home/vagrant/venv/lib/python3.8/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/home/vagrant/venv/lib/python3.8/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/vagrant/venv/lib/python3.8/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/home/vagrant/venv/lib/python3.8/site-packages/lingua/extract.py", line 426, in main
    for message in extractor(real_filename, extractor_options):
  File "/home/vagrant/venv/lib/python3.8/site-packages/lingua/extractors/python.py", line 415, in __call__
    return parser(token_stream, options, filename, lineno)
  File "/home/vagrant/venv/lib/python3.8/site-packages/lingua/extractors/python.py", line 188, in __call__
    self.process_token(token_type, token, location, token_stream)
  File "/home/vagrant/venv/lib/python3.8/site-packages/lingua/extractors/python.py", line 216, in process_token
    self.handler(token_type, token, location, token_stream)
  File "/home/vagrant/venv/lib/python3.8/site-packages/lingua/extractors/python.py", line 310, in state_in_keyword_call
    token = safe_eval(token)
  File "/home/vagrant/venv/lib/python3.8/site-packages/lingua/extractors/python.py", line 111, in safe_eval
    return ast.literal_eval(s)
  File "/usr/lib/python3.8/ast.py", line 99, in literal_eval
    return _convert(node_or_string)
  File "/usr/lib/python3.8/ast.py", line 98, in _convert
    return _convert_signed_num(node)
  File "/usr/lib/python3.8/ast.py", line 75, in _convert_signed_num
    return _convert_num(node)
  File "/usr/lib/python3.8/ast.py", line 66, in _convert_num
    _raise_malformed_node(node)
  File "/usr/lib/python3.8/ast.py", line 63, in _raise_malformed_node
    raise ValueError(f'malformed node or string: {node!r}')
ValueError: malformed node or string: <_ast.JoinedStr object at 0x7ffb86797880>

Any workaround for this besides changing f-strings to format() syntax?