Do not require typing_extensions at runtime

There are type annotations that indirectly depend on the
typing_extensions module (on Python 3.5-3.7: Protocol was added to the
core typing module in 3.8). The typing_extensions module is not
installed by default, so the code didn't run on a pristine Python
installation.

To avoid depending on a non-default module, make the dependency on
typing_extensions optional. (It's still required to run mypy, but
installing mypy takes care of providing typing_extensions.) If it
isn't available, provide a substitute definition that's just good
enough to get the scripts to run.

Move this ugly code to its own module to avoid the temptation of
spreading such ugliness all over the place. It's likely to be used in
other modules anyway.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/mbedtls_dev/test_case.py

@@ -19,15 +19,9 @@
 import binascii
 import os
 import sys
-from typing import Any, Iterable, List, Optional
-import typing_extensions #pylint: disable=import-error
-
-class Writable(typing_extensions.Protocol):
-    """Abstract class for typing hints."""
-    # pylint: disable=no-self-use,too-few-public-methods,unused-argument
-    def write(self, text: str) -> Any:
-        ...
+from typing import Iterable, List, Optional
 
+from mbedtls_dev import typing_util
 
 def hex_string(data: bytes) -> str:
     return '"' + binascii.hexlify(data).decode('ascii') + '"'
@@ -70,7 +64,7 @@ class TestCase:
         if self.function is None:
             raise MissingFunction
 
-    def write(self, out: Writable) -> None:
+    def write(self, out: typing_util.Writable) -> None:
         """Write the .data file paragraph for this test case.
 
         The output starts and ends with a single newline character. If the