.gitignore: Ignore Python 3 cache directories

These can appear when moving between branches that have different tools
in the tree. Ignore them.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

.gitignore

@@ -92,3 +92,6 @@ GTAGS
 *.orig
 *~
 \#*#
+
+# Python cache
+__pycache__