First, I attempted for "force inlining" (ex: __attribute__((always_inline)) for GCC/Clang), but it has been decided to not do that. Please read the discussion on the issue for the rationale.
It has been decided to use "static inline" syntax since it's the one required by the PEP 7, but I'm open to switch to "inline" (without static) or something else. Honestly, I don't understand exactly all consequences of the exact syntax, so I relied on other core devs who understand C99 that better than me :-)
Join the discussion on the issue ;-)