bpo-38270: Check for hash digest algorithms and avoid MD5 (GH-16382)

Make it easier to run and test Python on systems with restrict crypto policies:

* add requires_hashdigest to test.support to check if a hash digest algorithm is available and working
* avoid MD5 in test_hmac
* replace MD5 with SHA256 in test_tarfile
* mark network tests that require MD5 for MD5-based digest auth or CRAM-MD5


https://bugs.python.org/issue38270

Lib/test/test_imaplib.py

@@ -10,7 +10,8 @@ import threading
 import socket
 
 from test.support import (reap_threads, verbose, transient_internet,
-                          run_with_tz, run_with_locale, cpython_only)
+                          run_with_tz, run_with_locale, cpython_only,
+                          requires_hashdigest)
 import unittest
 from unittest import mock
 from datetime import datetime, timezone, timedelta
@@ -370,6 +371,7 @@ class NewIMAPTestsMixin():
         self.assertEqual(code, 'OK')
         self.assertEqual(server.response, b'ZmFrZQ==\r\n')  # b64 encoded 'fake'
 
+    @requires_hashdigest('md5')
     def test_login_cram_md5_bytes(self):
         class AuthHandler(SimpleIMAPHandler):
             capabilities = 'LOGINDISABLED AUTH=CRAM-MD5'
@@ -387,6 +389,7 @@ class NewIMAPTestsMixin():
         ret, _ = client.login_cram_md5("tim", b"tanstaaftanstaaf")
         self.assertEqual(ret, "OK")
 
+    @requires_hashdigest('md5')
     def test_login_cram_md5_plain_text(self):
         class AuthHandler(SimpleIMAPHandler):
             capabilities = 'LOGINDISABLED AUTH=CRAM-MD5'
@@ -797,6 +800,7 @@ class ThreadedNetworkedTests(unittest.TestCase):
                              b'ZmFrZQ==\r\n')  # b64 encoded 'fake'
 
     @reap_threads
+    @requires_hashdigest('md5')
     def test_login_cram_md5(self):
 
         class AuthHandler(SimpleIMAPHandler):