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_md.py b/test/py/tests/test_md.py
index 32cce4f..5fe2582 100644
--- a/test/py/tests/test_md.py
+++ b/test/py/tests/test_md.py
@@ -8,8 +8,8 @@
 
 @pytest.mark.buildconfigspec('cmd_memory')
 def test_md(u_boot_console):
-    '''Test that md reads memory as expected, and that memory can be modified
-    using the mw command.'''
+    """Test that md reads memory as expected, and that memory can be modified
+    using the mw command."""
 
     ram_base = u_boot_utils.find_ram_base(u_boot_console)
     addr = '%08x' % ram_base
@@ -24,8 +24,8 @@
 
 @pytest.mark.buildconfigspec('cmd_memory')
 def test_md_repeat(u_boot_console):
-    '''Test command repeat (via executing an empty command) operates correctly
-    for "md"; the command must repeat and dump an incrementing address.'''
+    """Test command repeat (via executing an empty command) operates correctly
+    for "md"; the command must repeat and dump an incrementing address."""
 
     ram_base = u_boot_utils.find_ram_base(u_boot_console)
     addr_base = '%08x' % ram_base