Matching the symbols permitted for x86_64 by GCC_4.3.0 require a limit of GCC_4.5.0 on i686 CentOS 6.
This does create the risk that 32-bit extension modules using `__extendxftf2` may not actually be portable to all 2010 era systems, but most of those other than RHEL/CentOS 6 have already hit the end of their respective support periods.
manylinux1 is getting old enough now to start making things
difficult (specifically around network security), so it's time
for a refresh to a slightly newer baseline.