refix py3.7 dict_keys not reversible bug

Author: tomchen

.vscode/settings.json

@@ -2,6 +2,5 @@
   "python.testing.unittestEnabled": false,
   "python.testing.nosetestsEnabled": false,
   "python.testing.pytestEnabled": true,
-  "python.envFile": "${workspaceRoot}/vscode.env",
-  "python.pythonPath": "C:\\Users\\Chen\\AppData\\Local\\Programs\\Python\\Python39\\python.exe"
+  "python.envFile": "${workspaceRoot}/vscode.env"
 }

src/bdfparser/bdfparser.py

@@ -379,7 +379,10 @@ def itercps(self, order=1, r=None):
         elif order == 2:
             retiterator = iter(sorted(ks, reverse=True))
         elif order == -1:
-            retiterator = reversed(ks)
+            try:
+                retiterator = reversed(ks)
+            except TypeError:
+                retiterator = reversed(list(ks))  # Python <=3.7
         if r is not None:
             def f(cp):
                 if isinstance(r, int):