Use new Poly1305 class methods from cryptography 2.8 release

This commit switches the Chacha20-Poly1305 code to use the one-line
class methods to generate and verify Poly1305 tags.

asyncssh/crypto/chacha.py

@@ -51,18 +51,13 @@ def poly1305_key(key, nonce):
     def poly1305(key, data, nonce):
         """Compute a Poly1305 tag for a block of data"""
 
-        poly = Poly1305(poly1305_key(key, nonce))
-        poly.update(data)
-        return poly.finalize()
+        return Poly1305.generate_tag(poly1305_key(key, nonce), data)
 
     def poly1305_verify(key, data, nonce, tag):
         """Verify a Poly1305 tag for a block of data"""
 
-        poly = Poly1305(poly1305_key(key, nonce))
-
         try:
-            poly.update(data)
-            poly.verify(tag)
+            Poly1305.verify_tag(poly1305_key(key, nonce), data, tag)
             return True
         except InvalidSignature:
             return False

setup.py

@@ -52,7 +52,7 @@
       long_description = long_description,
       platforms = 'Any',
       python_requires = '>= 3.6',
-      install_requires = ['cryptography >= 2.7'],
+      install_requires = ['cryptography >= 2.8'],
       extras_require = {
           'bcrypt':     ['bcrypt >= 3.1.3'],
           'gssapi':     ['gssapi >= 1.2.0'],