Fix tests with new Python

Author: shadchin

src/ecdsa/der.py

@@ -464,12 +464,11 @@ def unpem(pem):
     if isinstance(pem, text_type):  # pragma: no branch
         pem = pem.encode()
 
+    lines = (l.strip() for l in pem.split(b"\n"))
     d = b"".join(
-        [
-            l.strip()
-            for l in pem.split(b"\n")
-            if l and not l.startswith(b"-----")
-        ]
+        l
+        for l in lines
+        if l and not l.startswith(b"-----")
     )
     return base64.b64decode(d)