try importing blake2b from pyblake2 on ImportErrors

README.md

@@ -108,6 +108,10 @@ decently fast for example.
 Make sure you run a decently modern Python 3 (anything newer than `3.5` _should_ work, development
 was done on `3.7`).
 
+**Note:** When running on Python 3.5 (e.g. on Debian 9 "stretch") you need to install
+[pyblake2](https://pypi.org/project/pyblake2/) because the `hashlib` does not include `blake2b` in
+3.5 yet.
+
 ### PIP
 
 Install the package via `pip` by running:

imapfetch/imapfetch.py

@@ -6,7 +6,6 @@
 import imaplib
 import mailbox
 import logging
-import hashlib
 import configparser
 import time
 import binascii
@@ -15,6 +14,12 @@
 import os
 import re
 
+# import hashlib from library or pyblake2 package
+try:
+  from hashlib import blake2b
+except ImportError:
+  from pyblake2 import blake2b
+
 # system is windows
 nt = os.name == "nt"
 
@@ -29,7 +34,7 @@
 l = lambda n: log.getChild(n)
 
 # cryptographic hash for indexing purposes
-Blake2b = lambda b: hashlib.blake2b(b, digest_size=32).digest()
+Blake2b = lambda b: blake2b(b, digest_size=32).digest()
 
 # join path to absolute and expand ~ home
 join = lambda p, base=".": os.path.abspath(os.path.join(base, os.path.expanduser(p)))