cython: Explicitly set language level in .pyx files

[jvesely]

cython>=3 changes default language level, so stick with level=2

Closes: https://github.com/LBL-EESA/fastkde/issues/24

[jvesely]

The only language level issue is the division on line 161:

nspreadhalf = nspread/2

but setting the language level explicitly prevents possible future pitfalls

[taobrienlbl]

thank you so much @jvesely !!!