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>

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')