minor modernization

We still have code which checks if the hashlib module is present;
however hashlib is standard library since Python 2.5.

Hashlib (md5) is used to create something unique, and then format it
into a UUID (called GUID since this happens in a Windows context).
However, the uuid module is standard library since Python 2.5, and might
be more appropriate for the task.

There's a test which has this docstring:

"""
Verify falling back to 'timestamp' behavior if there is no native
hashlib and no underlying md5 module available.
"""

which thus is always skipped in our tests (in the md5-less FIPS
environment mentioned in email they might see this if they ran the tests).