gh-35091: fix and activate W391 in pyx files
    
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### 📚 Description


Fixes #34929

Activate the check for pycodestyle W391 in pyx files

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URL: #35091
Reported by: Frédéric Chapoton
Reviewer(s): Edgar Costa

Author: Release Manager

src/sage/algebras/fusion_rings/fast_parallel_fmats_methods.pyx

@@ -531,4 +531,3 @@ cdef pent_verify(factory, tuple mp_params):
             feq_verif(factory, worker_results, fvars, Nk_ij, id_anyon, nonuple)
         if i % 50000000 == 0 and i and verbose:
             print("{:5d}m equations checked... {} potential misses so far...".format(i // 1000000, len(worker_results)))
-

src/sage/algebras/fusion_rings/poly_tup_engine.pyx

@@ -576,4 +576,3 @@ cpdef tuple poly_tup_sortkey(tuple eq_tup):
             key.append(-exp._data[2*i])
             key.append(exp._data[2*i+1])
     return tuple(key)
-

src/sage/algebras/fusion_rings/shm_managers.pyx

@@ -773,4 +773,3 @@ def make_FvarsHandler(n, field, idx_map, init_data):
         sage: f.shutdown_worker_pool()
     """
     return FvarsHandler(n, field, idx_map, init_data=init_data)
-

src/sage/numerical/backends/generic_backend.pyx

@@ -1819,4 +1819,3 @@ cpdef GenericBackend get_solver(constraint_generation = False, solver = None, ba
 
     else:
         raise ValueError("'solver' should be set to 'GLPK', 'GLPK/exact', 'Coin', 'CPLEX', 'CVXOPT', 'Gurobi', 'PPL', 'SCIP', 'InteractiveLP', None (in which case the default one is used), or a callable.")
-

src/tox.ini

@@ -124,7 +124,7 @@ description =
     # See https://pycodestyle.pycqa.org/en/latest/intro.html#error-codes
 deps = pycodestyle
 commands = pycodestyle --select E111,E211,E306,E401,E701,E702,E703,W291,W391,W605,E711,E712,E713,E721,E722 {posargs:{toxinidir}/sage/}
-       pycodestyle --select E111,E306,E401,E703,W605,E712,E713,E721,E722 --filename *.pyx {posargs:{toxinidir}/sage/}
+       pycodestyle --select E111,E306,E401,E703,W391,W605,E712,E713,E721,E722 --filename *.pyx {posargs:{toxinidir}/sage/}
 
 [pycodestyle]
 max-line-length = 160