Replace cgi.parse_header() with custom function for Python 3.13 compat

[t-8ch]

The cgi module has been deprecated since Python 3.11 and got removed in Python 3.13. Replace the cgi.parse_header() function with a custom one, as recommended by the Python documentation.

Closes #712

Note: There is another usage of cgi in spyne/server/twisted/http.py, but I have no idea how twisted works, and it's not part of my usecase.

@bgermann Could you give this a try in the Debian packaging?

[bgermann]

With this, one test fails even with Python 3.12.

[t-8ch]

@bgermann Please try again

[hansnoir]

what's holding this one up?