Fix test_generators: save/restore warnings filters (#108246)

Previously, depending on existing filters, the test
could modify the warnings and so fail as "env changed".

Lib/test/test_generators.py

@@ -2258,6 +2258,7 @@ def printsolution(self, x):
 caught ValueError (xyz)
 
 >>> import warnings
+>>> old_filters = warnings.filters.copy()
 >>> warnings.filterwarnings("ignore", category=DeprecationWarning)
 
 # Filter DeprecationWarning: regarding the (type, val, tb) signature of throw().
@@ -2331,8 +2332,7 @@ def printsolution(self, x):
   ...
 ValueError: 7
 
->>> warnings.filters.pop(0)
-('ignore', None, <class 'DeprecationWarning'>, None, 0)
+>>> warnings.filters[:] = old_filters
 
 # Re-enable DeprecationWarning: the (type, val, tb) exception representation is deprecated,
 #                               and may be removed in a future version of Python.