Fix warnings (C and cython) #26

Job	Run time
cygwin (python-3, 9)	4m 2s
CI (macos-13 with Python 3.10)	1m 12s
CI (macos-13 with Python 3.11)	1m 22s
CI (macos-13 with Python 3.12)	1m 15s
CI (macos-13 with Python 3.13-dev)	1m 16s
CI (macos-latest with Python 3.10)	1m 31s
CI (macos-latest with Python 3.11)	1m 2s
CI (macos-latest with Python 3.12)	1m 1s
CI (macos-latest with Python 3.13-dev)	56s
CI (ubuntu-latest with Python 3.10)	54s
CI (ubuntu-latest with Python 3.11)	56s
CI (ubuntu-latest with Python 3.12)	56s
CI (ubuntu-latest with Python 3.13-dev)	1m 1s
CI (windows-latest with Python 3.10)	1m 36s
CI (windows-latest with Python 3.11)	1m 52s
CI (windows-latest with Python 3.12)	1m 51s
CI (windows-latest with Python 3.13-dev)	2m 10s
	24m 53s

Provide feedback