gnulib-tool.py: Don't use mutable default arguments.

[Collin Funk]

This patch fixes an interesting warning given by PyCharm.

With this line:

    def __init__(self, config: GLConfig, transformers: dict[str, 
tuple[re.Pattern, str] | None] = dict()) -> None:

Under '= dict()', I see a warning about mutable default arguments.
Here is a test program to demonstrate:

--------------------------------
#!/usr/bin/env python3

def function(arg1, arg2 = dict()):
    arg2[arg1] = 0
    print(arg2)

function('one')
function('two')
function('three')
--------------------------------

When executing the following is printed:

     {'one': 0}
     {'one': 0, 'two': 0}
     {'one': 0, 'two': 0, 'three': 0}

To avoid this behavior we can set the default value of 'transformers'
to None. Then in the body of __init__() we can set it to an empty
dictionary if it is None.

Collin

0001-gnulib-tool.py-Don-t-use-mutable-default-arguments.patch
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