Be more careful about reverting mutuations to system-wide (sys) variables.

This fixes 15 spurious test failures on Windows (probably all due to
the test leaving a wrong path in sys.argv[0], which then prevented
regrtest.py from finding the expected-output files for tests running
after test_optparse).

Lib/test/test_optparse.py

@@ -125,19 +125,25 @@ and kwargs %(kwargs)r
 
     def assertParseFail(self, cmdline_args, expected_output):
         """Assert the parser fails with the expected message."""
+        save_stderr = sys.stderr
+        try:
             sys.stderr = StringIO()
             self.assertRaises(self.parser.parse_args, (cmdline_args,), None,
                               SystemExit, expected_output,
                               self.redirected_stderr)
-        sys.stderr = sys.__stderr__
+        finally:
+            sys.stderr = save_stderr
 
     def assertStdoutEquals(self, cmdline_args, expected_output):
         """Assert the parser prints the expected output on stdout."""
+        save_stdout = sys.stdout
+        try:
             sys.stdout = StringIO()
             self.assertRaises(self.parser.parse_args, (cmdline_args,), None,
                               SystemExit, expected_output,
                               self.redirected_stdout)
-        sys.stdout = sys.__stdout__
+        finally:
+            sys.stdout = save_stdout
 
     def assertTypeError(self, func, expected_output, *args):
         """Assert a TypeError is raised when executing func."""
@@ -426,6 +432,9 @@ class TestProgName(BaseTest):
 
     def test_default_progname(self):
         # Make sure that program name taken from sys.argv[0] by default.
+        save_argv = sys.argv[:]
+        try:
+            # XXX Should the path be hard-coding forward-slashes?
             sys.argv[0] = "/foo/bar/baz.py"
             parser = OptionParser("usage: %prog ...", version="%prog 1.2")
             expected_usage = "usage: baz.py ...\n"
@@ -436,6 +445,8 @@ class TestProgName(BaseTest):
                 "options:\n"
                 "  --version   show program's version number and exit\n"
                 "  -h, --help  show this help message and exit\n")
+        finally:
+            sys.argv[:] = save_argv
 
     def test_custom_progname(self):
         parser = OptionParser(prog="thingy",