Add support for `mode="x"` in `SevenZipFile()` constructor

[Ravencentric]

Is your feature request related to a problem? Please describe. SevenZipFile does not support mode="x" which stands for "exclusive writing mode".

The official documentation for open() states open for exclusive creation, failing if the file already exists.

From my understanding:

w	x
open for writing	open for writing
create the file if it doesn't exist	create the file if it doesn't exist
Overwrite the file if it exists	raise FileExistsError if the file exists

Describe the solution you'd like Considering that w and x are near identical, minimal code changes are required. In fact, seems to me py7zr already has code in place to handle it?

https://github.com/miurahr/py7zr/blob/606b50ea2561601c59a1232b17dec43416f61bab/py7zr/py7zr.py#L360-L361 https://github.com/miurahr/py7zr/blob/606b50ea2561601c59a1232b17dec43416f61bab/py7zr/py7zr.py#L374-L375

But it fails here: https://github.com/miurahr/py7zr/blob/606b50ea2561601c59a1232b17dec43416f61bab/py7zr/py7zr.py#L343-L344

Describe alternatives you've considered Manually check for existing file before opening in mode="w".

Additional context N/A