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-gerrit b/amboar/obmc-scripts/maintainers/obmc-gerrit
index 800e561..e7e8f6a 100755
--- a/amboar/obmc-scripts/maintainers/obmc-gerrit
+++ b/amboar/obmc-scripts/maintainers/obmc-gerrit
@@ -17,7 +17,7 @@
 git = sh.git.bake()
 
 def get_reviewers(root: Optional[str]=None, mname: str='MAINTAINERS') -> List[str]:
-    reviewers: List[str] = list()
+    reviewers = cast(List[str], list())
     if not root:
         root = git('rev-parse', '--show-toplevel').strip()
     mfile = os.path.join(root, mname)