Fix setup link in README

[Tom-Hubrecht]

It was removed in
73fbb62

[oscarbenjamin]

Thanks

[oscarbenjamin]

The CI job that failed was testing against FLINT main branch.

Something must have changed very recently but the build failure is:

FAILED: src/flint/types/nmod_mat.cpython-313-darwin.so.p/meson-generated_src_flint_types_nmod_mat.pyx.c.o
cc -Isrc/flint/types/nmod_mat.cpython-313-darwin.so.p -Isrc/flint/types -I../src/flint/types -I/install/include/python3.13 -I/Users/enojb/.local/share/uv/python/cpython-3.13.0-macos-aarch64-none/include/python3.13 -I/Users/enojb/work/dev/python-flint/.local/include -fvisibility=hidden -fdiagnostics-color=always -Wall -Winvalid-pch -O0 -g -MD -MQ src/flint/types/nmod_mat.cpython-313-darwin.so.p/meson-generated_src_flint_types
_nmod_mat.pyx.c.o -MF src/flint/types/nmod_mat.cpython-313-darwin.so.p/meson-generated_src_flint_types_nmod_mat.pyx.c.o.d -o src/flint/types/nmod_mat.cpython-313-darwin.so.p/meson-generated_src_flint_types_nmod_mat.pyx.c.o -c src/flint/types/nmod_mat.cpython-313-darwin.so.p/src/flint/types/nmod_mat.pyx.c
src/flint/types/nmod_mat.cpython-313-darwin.so.p/src/flint/types/nmod_mat.pyx.c:5632:32: error: no member named 'rows' in 'nmod_mat_struct'
          ((__pyx_v_self->val->rows[__pyx_v_i])[__pyx_v_j]) = __pyx_t_4;
            ~~~~~~~~~~~~~~~~~  ^
src/flint/types/nmod_mat.cpython-313-darwin.so.p/src/flint/types/nmod_mat.pyx.c:5898:30: error: no member named 'rows' in 'nmod_mat_struct'
        ((__pyx_v_self->val->rows[__pyx_v_i])[__pyx_v_j]) = __pyx_t_4;
          ~~~~~~~~~~~~~~~~~  ^

Looks like nmod_mat_struct was changed in flintlib/flint#2162

Maybe the more stable solution is to use nmod_mod_mat_set_entry e.g. here:

python-flint/src/flint/types/nmod_mat.pyx

Line 122 in 9c9a7d1

self.val.rows[i][j] = (<nmod>x).val