add cython nogil arg None (plumed#1241)

Author: jamesmkrieger

python/plumed.pyx

@@ -187,7 +187,7 @@ cdef class Plumed:
          nothrow.ptr=&error
          nothrow.handler=cplumed.plumed_error_set
          # see https://github.com/plumed/plumed2/pull/1129#issuecomment-2410867829
-         with cython.nogil():
+         with cython.nogil:
             cplumed.plumed_cmd_safe_nothrow(self.c_plumed,ckey,safe,nothrow)
          if(error.code):
            try: