swh.model.hashutil: Use pyblake2 dependency on python3 <= 3.4

This resolves the caveat mentioned in prior commit about not being
able to use blake2 prior to 3.5

Related T692
Closes D192

debian/control

@@ -7,6 +7,7 @@ Build-Depends: debhelper (>= 9),
                python3-all,
                python3-nose,
                python3-setuptools,
+               python3-pyblake2,
                python3-vcversioner
 Standards-Version: 3.9.6
 Homepage: https://forge.softwareheritage.org/diffusion/DMOD/

requirements-swh.txt

+pyblake2

swh/model/hashutil.py

@@ -28,6 +28,7 @@ import binascii
 import functools
 import hashlib
 import os
+import sys
 
 from io import BytesIO
 
@@ -41,6 +42,15 @@ DEFAULT_ALGORITHMS = set(['sha1', 'sha256', 'sha1_git'])
 # FWIW coreutils' sha1sum uses 32768
 HASH_BLOCK_SIZE = 32768
 
+# Prior to python3.4, only blake2 is available through pyblake2 module
+# From 3.5 onwards, it's been integrated in python
+if sys.version_info.major == 3 and sys.version_info.minor <= 4:
+    import pyblake2
+    # register those hash algorithms in hashlib
+    __cache = hashlib.__builtin_constructor_cache
+    __cache['blake2s256'] = pyblake2.blake2s
+    __cache['blake2b512'] = pyblake2.blake2b
+
 
 def _new_git_hash(base_algo, git_type, length):
     """Initialize a digest object (as returned by python's hashlib) for the