maintainers: Avoid backwards-incompatible type declarations

Local-variable type declarations break on interpreter versions < 3.6.
Instead, cast the object to the type to avoid the incompatible syntax.

Change-Id: I1b813908002fe6aac95e6514ac09838d2f28daa6
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/amboar/obmc-scripts/maintainers/obmc/maintainers.py b/amboar/obmc-scripts/maintainers/obmc/maintainers.py
index d18d676..2b261af 100755
--- a/amboar/obmc-scripts/maintainers/obmc/maintainers.py
+++ b/amboar/obmc-scripts/maintainers/obmc/maintainers.py
@@ -106,7 +106,7 @@
 
 def parse_block(src: Iterator[str]) -> Optional[B]:
     state = ParseState.BEGIN
-    repo: Dict[LineType, D] = OrderedDict()
+    repo = cast(B, OrderedDict())
     for line in src:
         try:
             entry = parse_line(line)
@@ -149,10 +149,10 @@
             break
 
 def parse_maintainers(src: Iterator[str]) -> Dict[D, B]:
-    maintainers: Dict[D, B] = OrderedDict()
+    maintainers = cast(Dict[D, B], OrderedDict())
     trash_preamble(src)
     while True:
-        repo: B = parse_block(src)
+        repo = cast(B, parse_block(src))
         if not repo:
             break
         maintainers[repo[LineType.REPO]] = repo