Split Generators tests from Sequence tests (keras-team#11901)

* Split Sequence test from generator tests

* use_spawn check for windows

* Do not copy use_spawn

* Remove unused param

tests/keras/utils/data_utils_test.py

@@ -13,7 +13,6 @@
 import six
 from six.moves.urllib.parse import urljoin
 from six.moves.urllib.request import pathname2url
-import warnings
 
 from flaky import flaky
 
@@ -33,17 +32,15 @@
                                      'TRAVIS_PYTHON_VERSION' in os.environ,
                                      reason='Generators do not work with `spawn`.')
 
-if sys.version_info < (3,):
-    def next(x):
-        return x.next()
-
 
 def use_spawn(func):
-    """Decorator to test both Unix (fork) and Windows (spawn)"""
+    """Decorator which uses `spawn` when possible.
+    This is useful on Travis to avoid memory issues.
+    """
 
     @six.wraps(func)
     def wrapper(*args, **kwargs):
-        if sys.version_info > (3, 4):
+        if sys.version_info > (3, 4) and os.name != 'nt':
             mp.set_start_method('spawn', force=True)
             out = func(*args, **kwargs)
             mp.set_start_method('fork', force=True)
@@ -54,6 +51,11 @@ def wrapper(*args, **kwargs):
     return wrapper
 
 
+if sys.version_info < (3,):
+    def next(x):
+        return x.next()
+
+
 @pytest.fixture
 def in_tmpdir(tmpdir):
     """Runs a function in a temporary directory.