hexdigest() should return a string object not bytes

For consistency with the hashlib module in the standard library.

Fixes #6.

Author: dnicolodi

docs/index.rst

@@ -81,7 +81,7 @@ methods provided by the hash objects in the standard library
 
 .. method:: hash.hexdigest()
 
-   Like :meth:`digest` except the digest is returned as a bytes object
+   Like :meth:`digest` except the digest is returned as a string object
    of double length, containing only hexadecimal digits.
 
 .. method:: hash.copy()

siphash24.pyx.in

@@ -32,7 +32,7 @@ cdef bytes uint64le(uint64_t v):
     return <bytes>r[:8]
 
 
-cdef bytes hexlify(uint64_t v):
+cdef str hexlify(uint64_t v):
     cdef char string[17]
     PyOS_snprintf(
         string, sizeof(string),
@@ -45,7 +45,7 @@ cdef bytes hexlify(uint64_t v):
         <uint8_t>(v >> 40),
         <uint8_t>(v >> 48),
         <uint8_t>(v >> 56))
-    return <bytes>string[:16]
+    return string[:16].decode('ascii')
 
 
 {{for variant in "13", "24"}}

test.py

@@ -32,7 +32,7 @@ def test_siphash24_data_and_key(self):
     def test_siphash24_hexdigest(self):
         h = siphash24.siphash24()
         self.assertIsInstance(h.digest(), bytes)
-        self.assertEqual(h.hexdigest(), binascii.hexlify(h.digest()))
+        self.assertEqual(h.hexdigest(), binascii.hexlify(h.digest()).decode('ascii'))
 
     def test_siphash24_name(self):
         h = siphash24.siphash24()