support old Python without typing.Protocol

pdfminer/pdftypes.py

@@ -1,7 +1,8 @@
 import zlib
 import logging
-from typing import (TYPE_CHECKING, Any, Dict, Iterable, Optional, Protocol,
-                    Union, List, Tuple, cast)
+import sys
+from typing import (TYPE_CHECKING, Any, Dict, Iterable, Optional, Union, List,
+                    Tuple, cast)
 from .lzw import lzwdecode
 from .ascii85 import ascii85decode
 from .ascii85 import asciihexdecode
@@ -32,10 +33,16 @@
 LITERALS_JBIG2_DECODE = (LIT('JBIG2Decode'),)
 
 
-class DecipherCallable(Protocol):
-    def __call__(self, objid: int, genno: int, data: bytes,
-                 attrs: Optional[Dict[str, Any]] = None) -> bytes:
-        raise NotImplementedError
+if sys.version_info >= (3, 8):
+    from typing import Protocol
+    class DecipherCallable(Protocol):
+        """Fully typed a decipher callback, with optional parameter."""
+        def __call__(self, objid: int, genno: int, data: bytes,
+                    attrs: Optional[Dict[str, Any]] = None) -> bytes:
+            raise NotImplementedError
+else:  # Fallback for older Python
+    from typing import Callable
+    DecipherCallable = Callable[..., bytes]
 
 
 class PDFObject(PSObject):