binman: Fix typing for python >= 3.7

To get the [] annotation working with python 3.7 and 3.8, import
annotations.

Reported-by: Tim Harvey <tharvey@gateworks.com>
Fixes: 21bc3433a4 ("binman: rework dropping absent entries from packaged image")
Reviewed-by: Bryan Brattlof <bb@ti.com>
Signed-off-by: Yannic Moog <y.moog@phytec.de>
Tested-by: Tim Harvey <tharvey@gateworks.com>

tools/binman/etype/cbfs.py

@@ -5,6 +5,7 @@
 # Entry-type module for a Coreboot Filesystem (CBFS)
 #
 
+from __future__ import annotations
 from collections import OrderedDict
 
 from binman import cbfs_util

tools/binman/etype/mkimage.py

@@ -5,6 +5,7 @@
 # Entry-type module for producing an image using mkimage
 #
 
+from __future__ import annotations
 from collections import OrderedDict
 
 from binman.entry import Entry

tools/binman/etype/section.py

@@ -8,6 +8,7 @@ Sections are entries which can contain other entries. This allows hierarchical
 images to be created.
 """
 
+from __future__ import annotations
 from collections import OrderedDict
 import concurrent.futures
 import re