s = b'\x03\xc4\x85v\x00\x00\x00\x00'

	    def ashex(s):
	>       return s.encode('hex')
	E       AttributeError: 'bytes' object has no attribute 'encode'

	zodbtools/util.py:29: AttributeError

s.encode('hex') used to work on Py2 but fails on Py3:

	In [1]: s = "abc"

	In [2]: b = b"def"

	In [3]: s.encode('hex')
	---------------------------------------------------------------------------
	LookupError                               Traceback (most recent call last)
	<ipython-input-3-75ae843597fe> in <module>()
	----> 1 s.encode('hex')

	LookupError: 'hex' is not a text encoding; use codecs.encode() to handle arbitrary codecs

	In [4]: b.encode('hex')
	---------------------------------------------------------------------------
	AttributeError                            Traceback (most recent call last)
	<ipython-input-4-ec2fccff20bc> in <module>()
	----> 1 b.encode('hex')

	AttributeError: 'bytes' object has no attribute 'encode'

	In [5]: import codecs

	In [6]: codecs.encode(b, 'hex')
	Out[6]: b'646566'

	In [7]: codecs.encode(s, 'hex')
	---------------------------------------------------------------------------
	TypeError                                 Traceback (most recent call last)
	/usr/lib/python3.7/encodings/hex_codec.py in hex_encode(input, errors)
	     14     assert errors == 'strict'
	---> 15     return (binascii.b2a_hex(input), len(input))
	     16

	TypeError: a bytes-like object is required, not 'str'

	The above exception was the direct cause of the following exception:

	TypeError                                 Traceback (most recent call last)
	<ipython-input-7-7fcb16cead4f> in <module>()
	----> 1 codecs.encode(s, 'hex')

	TypeError: encoding with 'hex' codec failed (TypeError: a bytes-like object is required, not 'str')

After the patch it works with bytes and raises for str.
Fromhex does not need to be changed - it already uses codecs.decode way as
originally added in dd959b28 (zodbdump += DumpReader - to read/parse zodbdump
stream).

Based on patch by Jérome Perrin.