Maybe, two macros can be useful for everyone:

* the path suffix
* the suffix used in lib-Requires (in parentheses).

This code only defines the latter one -- http://git.altlinux.org/gears/r/rpm-build-python3.git?p=rpm-build-python3.git;a=blob;f=rpm-build-python3/python3;h=ff38c643b7f54414e8534424e067352b7fd0021a;hb=HEAD#l170 :

# What follows next is a computation of the suffix, which is
# new-arch-ABI-proof and which can be generally useful in all RPM specs
# (not python-specific).
#
# If a new weird arch-ABI arises, at least, we'll have
# a weird suffix here, not coinciding with another existing one:
#
# if "%_lib" == "lib", then empty string;
# otherwise, for example,
# if "%_lib" == "lib64", then "(64bit)" (without the quotation marks).
%ABI_suffix %(s="%_lib"; s=${s#lib}; echo "${s:+(${s}bit)}")

Let's move it from rpm-build-python3 to rpm-build.