patman: Rename 'str' variable in EmailPatches()

This is not a good variable name in Python because 'str' is a type. It
shows up highlighted in some editors. Rename it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
1 file changed