gh-155212: List the converters defined in a C file

[serhiy-storchaka]

--converters now accepts file names and can be used with --make.
It parses the specified files, without writing anything, and prints the converters and return converters which they define, instead of the built-in ones.

$ ./python Tools/clinic/clinic.py --converters Modules/posixmodule.c

Legacy converters:
    " _Py_PARSE_INTPTR " " _Py_PARSE_PID "

Converters:
    confname(table)
    dev_t()
    dir_fd()
    ...
    path_t(allow_fd=False, make_wide=None, nonstrict=False, nullable=False, suppress_value_error=False)

Return converters:
    dev_t()

Two other changes:

• Legacy converters whose format unit does not start with a letter, like those defined in Modules/posixmodule.c, were silently omitted from the list.
• A directory passed as a file name raised IsADirectoryError instead of reporting an error, in every mode.

• Issue: Argument Clinic: list the converters defined in a C file #155212