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_sleep.py

@@ -6,8 +6,8 @@ import pytest
 import time
 
 def test_sleep(u_boot_console):
-    '''Test the sleep command, and validate that it sleeps for approximately
-    the correct amount of time.'''
+    """Test the sleep command, and validate that it sleeps for approximately
+    the correct amount of time."""
 
     # 3s isn't too long, but is enough to cross a few second boundaries.
     sleep_time = 3