Modern CMake Build fix

[croi900]

The current modern CMake versions cannot build the package because of the cmake_minimum_required(VERSION 3.0).

Changing it to be >= 3.6 will allow modern toolchains to build the package, making it usable again.

Moreover, when attempting to build following the version change, on gcc 15.2.1 20251112, the following error occured:

NumbaQuadpack/src/dqc25c.c:51:23: error: passing argument 2 of ‘G_K15W’ from incompatible pointer type [-Wincompatible-pointer-types]
   51 |     result = G_K15W(f,dqwgtc,c,p2,p3,p4,kp,a,b,abserr,&resabs,&resasc, user_data);
      |                       ^~~~~~
      |                       |
      |                       double (*)(double,  double,  double,  double,  double,  int)

by introducing dq_weight_function_type this error no longer occurs and the library builds, so does the python package.