zerver/lib/initial_password.py: Encode return value.

zerver.lib.initial_password.initial_password is supposed to return an
Optional[text_type], but it returns an Optional[binary_type] instead.
Encode the return value to make sure it returns an Optional[text_type].

zerver/lib/initial_password.py

@@ -17,7 +17,7 @@ def initial_password(email):
     if settings.INITIAL_PASSWORD_SALT is not None:
         encoded_key = (settings.INITIAL_PASSWORD_SALT + email).encode("utf-8")
         digest = hashlib.sha256(encoded_key).digest()
-        return base64.b64encode(digest)[:16]
+        return base64.b64encode(digest)[:16].decode('utf-8')
     else:
         # None as a password for a user tells Django to set an unusable password
         return None