Fix a few uncommon pylint errors in compat.py

- It's ok if the optional modules functools, roman and hashlib are not there.
- It's ok to reference any and all before defining them. We're doing all
  this exactly because, under python 2.4, they are undefined.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>