gh-39010: Don't import cysignals.alarm on Windows
    
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`cysignals.alarm` is not available on Windows, so we don't try to import
it. Probably more work needs to be done to actually fix the methods that
make use of these methods on Windows; but that's left for further PRs.

Also remove a `cysignals` import from an `all_xyz.py` file that was
duplicated in the main `all.py` file.

### 📝 Checklist

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- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: #39010
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

Author: Release Manager

src/sage/all.py

@@ -57,6 +57,7 @@
 import os
 import operator
 import math
+import sys
 
 # ############### end setup warnings ###############################
 
@@ -157,7 +158,8 @@
 
 from sage.manifolds.all import *
 
-from cysignals.alarm import alarm, cancel_alarm
+if sys.platform != 'win32':
+    from cysignals.alarm import alarm, cancel_alarm
 
 # Lazily import interacts (#15335)
 lazy_import('sage.interacts', 'all', 'interacts')

src/sage/all__sagemath_objects.py

@@ -21,8 +21,6 @@
 
 from sage.cpython.all import *
 
-from cysignals.alarm import alarm, cancel_alarm
-
 from copy import copy, deepcopy
 
 true = True

src/sage/matrix/benchmark.py

@@ -18,13 +18,17 @@
     ======================================================================
 """
 
-from .constructor import random_matrix, Matrix
+import sys
+
+from sage.matrix.constructor import Matrix, random_matrix
 from sage.misc.lazy_import import lazy_import
+from sage.misc.timing import cputime
+from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
 from sage.rings.integer_ring import ZZ
 from sage.rings.rational_field import QQ
-from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
-from sage.misc.timing import cputime
-from cysignals.alarm import AlarmInterrupt, alarm, cancel_alarm
+
+if sys.platform != 'win32':
+    from cysignals.alarm import AlarmInterrupt, alarm, cancel_alarm
 
 lazy_import('sage.interfaces.magma', 'magma')
 

src/sage/parallel/use_fork.py

@@ -13,8 +13,11 @@
 # ****************************************************************************
 
 
+import sys
 from shutil import rmtree
-from cysignals.alarm import AlarmInterrupt, alarm, cancel_alarm
+
+if sys.platform != 'win32':
+    from cysignals.alarm import AlarmInterrupt, alarm, cancel_alarm
 
 from sage.interfaces.process import ContainChildren
 from sage.misc.timing import walltime

src/sage/tests/benchmark.py

@@ -15,29 +15,33 @@
 
     sage: import sage.tests.benchmark
 """
-from cysignals.alarm import alarm, cancel_alarm, AlarmInterrupt
+import sys
+
+if sys.platform != 'win32':
+    from cysignals.alarm import AlarmInterrupt, alarm, cancel_alarm
+
 from sage.combinat.combinat import fibonacci
 from sage.functions.other import factorial
 from sage.interfaces.gp import gp
 from sage.interfaces.macaulay2 import macaulay2
-from sage.interfaces.magma import magma, Magma
+from sage.interfaces.magma import Magma, magma
 from sage.interfaces.maple import maple
 from sage.interfaces.mathematica import mathematica
 from sage.interfaces.maxima import maxima
 from sage.interfaces.singular import singular
-from sage.libs.pari import pari
 from sage.libs.gap.libgap import libgap
+from sage.libs.pari import pari
 from sage.matrix.matrix_space import MatrixSpace
 from sage.misc.functional import log
 from sage.misc.timing import cputime, walltime
 from sage.modular.modsym.modsym import ModularSymbols
 from sage.rings.complex_mpfr import ComplexField
 from sage.rings.finite_rings.finite_field_constructor import GF
 from sage.rings.finite_rings.integer_mod_ring import Integers
-from sage.rings.rational_field import QQ
 from sage.rings.integer import Integer
 from sage.rings.integer_ring import ZZ
 from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
+from sage.rings.rational_field import QQ
 from sage.schemes.elliptic_curves.constructor import EllipticCurve