set encoding before passing to hash function

This fixes:

    $ vault_from_gpg_agent.py --clear
    Traceback (most recent call last):
      File "/home/tpo/bin/vault_from_gpg_agent.py", line 88, in <module>
        main()
      File "/home/tpo/bin/vault_from_gpg_agent.py", line 79, in main
        hashed_path = base64.b64encode(hashlib.sha1(my_path).hexdigest())
    TypeError: Unicode-objects must be encoded before hashing

I do `my_path.encode('utf-8')` assuming (hoping) that
`vault_from_gpg_agent.py`'s path on a system will not be changing over
time with respect to encoding. That means `stat $my_path` will always
return the same string with the same encoding. If that remains true then
I think we can encode `my_path` as we wish, provided that utf-8 can hold
all the characters that can occur in a filename...

Author: tpo

vault_from_gpg_agent.py

@@ -72,7 +72,7 @@ def main():
     my_path = os.path.realpath(sys.argv[0])
     # Per the source, cache-id is limited to 50 bytes, so we hash our
     # path and Base64 encode the path.
-    hashed_path = base64.b64encode(hashlib.sha1(my_path).digest())
+    hashed_path = base64.b64encode(hashlib.sha1(my_path.encode('utf-8')).digest())
     cache_id = "ansible-vault:%s" % (hashed_path,)
     if args.clear:
         clear_passphrase(gpg_agent, cache_id)