utils: Initialize hashers with bytes

	self = <zodbtools.util.CRC32Hasher object at 0x7f887ae465f8>

	    def __init__(self):
	>       self._h = crc32('')
	E       TypeError: a bytes-like object is required, not 'str'

	util.py:208: TypeError

Based on patch by Jérome Perrin.

zodbtools/util.py

@@ -188,7 +188,7 @@ class Adler32Hasher:
     digest_size = 4
 
     def __init__(self):
-        self._h = adler32('')
+        self._h = adler32(b'')
 
     def update(self, data):
         self._h = adler32(data, self._h)
@@ -205,7 +205,7 @@ class CRC32Hasher:
     digest_size = 4
 
     def __init__(self):
-        self._h = crc32('')
+        self._h = crc32(b'')
 
     def update(self, data):
         self._h = crc32(data, self._h)