Add a mypy.ini file and enable some more warnings.

Also require all Python functions to have a type declaration.

lib/cretonne/meta/build.py

@@ -18,7 +18,7 @@ parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
 parser.add_argument('--out-dir', help='set output directory')
 
 args = parser.parse_args()
-out_dir = args.out_dir  # type: ignore
+out_dir = args.out_dir
 
 isas = isa.all_isas()