MNT log1p -> log in cython files

[qinhanmin2014]

Revert the cython part of #11428
Will merge when green.

[rth]

With respect to the error mentioned in #11848, I think the issue is that MSVC didn't include log1p in older versions (cf e.g. rust-lang/rust#29479). This is something we can revert once python 2 support is dropped (as Python 3 uses a newer version of MSVC on windows).

[rth]

Thanks !

[jakirkham]

FWIW NumPy handles the problem by defining log1p when it is not available. Cython often exports NumPy C functions like this. So we could use those instead to get the best of both worlds (e.g. log1p when system provided and a custom C function when not).