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_unknown_cmd.py

@@ -4,8 +4,8 @@
 # SPDX-License-Identifier: GPL-2.0
 
 def test_unknown_command(u_boot_console):
-    '''Test that executing an unknown command causes U-Boot to print an
-    error.'''
+    """Test that executing an unknown command causes U-Boot to print an
+    error."""
 
     # The "unknown command" error is actively expected here,
     # so error detection for it is disabled.