add test coverage for the str() case in unpem

Author: tomato42

src/ecdsa/test_der.py

@@ -26,6 +26,8 @@
     remove_octet_string,
     remove_sequence,
     encode_implicit,
+    unpem,
+    topem,
 )
 
 
@@ -629,3 +631,9 @@ def test_remove_implicit_rejects_truncated_length():
         UnexpectedDER, match="Length longer than the provided buffer"
     ):
         remove_implicit(bad)
+
+
+def test_unpem_with_str():
+    data = b"\xff\x00\x01"
+    out = unpem(str(topem(data, "KEY"), "ASCII"))
+    assert data == out