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_urllib2_localnet.py

@@ -322,6 +322,7 @@ class ProxyAuthTests(unittest.TestCase):
     PASSWD = "test123"
     REALM = "TestRealm"
 
+    @support.requires_hashdigest("md5")
     def setUp(self):
         super(ProxyAuthTests, self).setUp()
         # Ignore proxy bypass settings in the environment.