Problem with cffi dependency

[mxmlnkn]

Hello,

I'm trying to use your module for ratarmount as a backend allowing me to seek inside xz compressed tar archives.

However, when I try to install it from pip, I get an error about the missing cffi module during wheel building.
I'm not sure but I think the problem might be that cffi is only listed in install_requires and not also in setup_requires because I see cffi being collected but not yet installed when trying to build the lzmaffi wheel.

Alternatively, I think it would be best to upload manylinux wheels in order to avoid requiring the user to have compatible build tools.