Add Python 3.13 support using `legacy-cgi` #106

Job	Run time
Lint the package	27s
Validate coverage	31s
Build the documentation	25s
Python: 3.8- on ubuntu-latest	23s
Python: 3.8- on windows-latest	1m 20s
Python: 3.8- on macos-latest	50s
Python: 3.9- on ubuntu-latest	26s
Python: 3.9- on windows-latest	1m 14s
Python: 3.9- on macos-latest	26s
Python: 3.10- on ubuntu-latest	32s
Python: 3.10- on windows-latest	1m 23s
Python: 3.10- on macos-latest	24s
Python: 3.11- on ubuntu-latest	26s
Python: 3.11- on windows-latest	1m 19s
Python: 3.11- on macos-latest	37s
Python: 3.12- on ubuntu-latest	29s
Python: 3.12- on windows-latest	1m 19s
Python: 3.12- on macos-latest	24s
Python: 3.13- on ubuntu-latest	36s
Python: 3.13- on windows-latest	2m 15s
Python: 3.13- on macos-latest	40s
Python: pypy-3.9- on ubuntu-latest	42s
Python: pypy-3.10- on ubuntu-latest	43s
Python: pypy-3.10- on windows-latest	2m 12s
Python: pypy-3.10- on macos-latest	1m 3s
	21m 6s

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