Use no_gc for nmod

Author: oscarbenjamin

src/flint/types/nmod.pxd

@@ -23,6 +23,7 @@ cdef class nmod_ctx:
     cdef nmod new_nmod(self)
 
 
+@cython.no_gc
 cdef class nmod(flint_scalar):
     cdef mp_limb_t val
     cdef nmod_ctx ctx

src/flint/types/nmod.pyx

@@ -22,6 +22,7 @@ from flint.utils.flint_exceptions import DomainError
 cdef dict _nmod_ctx_cache = {}
 
 
+@cython.no_gc
 cdef class nmod_ctx:
     """
     Context object for creating :class:`~.nmod` initalised 
@@ -163,6 +164,7 @@ cdef class nmod_ctx:
         return r
 
 
+@cython.no_gc
 cdef class nmod(flint_scalar):
     """
     The nmod type represents elements of Z/nZ for word-size n.