f-string parser: Fix error messages for test_conversions

[pablogsal]

In test_fstring the test_conversions test is failing due to what seems a mismatched error message but it may very well be a problem in the tokenizer. We need to investigate and either fix the tokenizer or update the test error messages. Example failure:

FAIL: test_conversions (test.test_fstring.TestCase) (str="f'{3!ss:s}'")\n----------------------------------------------------------------------
  File "<string>", line 1
    f'{3!ss:s}'
         ^
SyntaxError: Invalid conversion character

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/pablogsal/github/python/f-string-grammar/Lib/test/test_fstring.py", line 32, in assertAllRaise
    with self.assertRaisesRegex(exception_type, regex):
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: "f-string: expecting '}'" does not match "Invalid conversion character (<string>, line 1)"