fix!: drop support for Python 2.7 (#778)

Drop use of 'six' wrapper library.

Drop 'u"' prefixes.

Drop support for app_engine 'classic' mode (Python 2.7-only).

Release-As: 2.0.0b1

Closes #777.
diff --git a/tests/crypt/test_es256.py b/tests/crypt/test_es256.py
index 5bb9050..720f74c 100644
--- a/tests/crypt/test_es256.py
+++ b/tests/crypt/test_es256.py
@@ -50,7 +50,7 @@
 
 
 class TestES256Verifier(object):
-    def test_verify_success(self):
+    def test_verify_bytes_success(self):
         to_sign = b"foo"
         signer = es256.ES256Signer.from_string(PRIVATE_KEY_BYTES)
         actual_signature = signer.sign(to_sign)
@@ -58,8 +58,8 @@
         verifier = es256.ES256Verifier.from_string(PUBLIC_KEY_BYTES)
         assert verifier.verify(to_sign, actual_signature)
 
-    def test_verify_unicode_success(self):
-        to_sign = u"foo"
+    def test_verify_text_success(self):
+        to_sign = "foo"
         signer = es256.ES256Signer.from_string(PRIVATE_KEY_BYTES)
         actual_signature = signer.sign(to_sign)