test/py: use " for docstrings

Python's coding style docs indicate to use " not ' for docstrings.

test/py has other violations of the coding style docs, since the docs
specify a stranger style than I would expect, but nobody has complained
about those yet:-)

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_help.py b/test/py/tests/test_help.py
index 894f3b5..420090c 100644
--- a/test/py/tests/test_help.py
+++ b/test/py/tests/test_help.py
@@ -4,6 +4,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
 def test_help(u_boot_console):
-    '''Test that the "help" command can be executed.'''
+    """Test that the "help" command can be executed."""
 
     u_boot_console.run_command('help')