py3c: unconditionally drop -Werror (fix gcc-11 build)
py3c: unconditionally drop -Werror (fix gcc-11 build)

On gcc-11 build fails as:

    $ nix-build -E 'with import ./. {}; py3c.override { stdenv = gcc11Stdenv; }'

    gcc -fno-strict-aliasing ... -Werror -Wall
    cc1plus: warning:
      command-line option '-Wstrict-prototypes' is valid for C/ObjC but not for C++
    In file included from /nix/store/...-python-2.7.18/include/python2.7/Python.h:88,
                     from test_py3c.cpp:4:
    /nix/store/...-python-2.7.18/include/python2.7/unicodeobject.h:534:24:
      error: ISO C++17 does not allow 'register' storage class specifier [-Werror=register]
      534 |     register PyObject *obj,     /* Object */
          |                        ^~~

The fix was already present o clang toolchain. Let's extendt it to all toolchains.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs