util: don't use mutable default argument value
I don't think this is any tight loops and we'd need to worry about
PyObject creation overhead. Also, I'm pretty sure strptime()
will be much slower than PyObject creation (date parsing is
surprisingly slow).
match: don't use mutable default argument value
There shouldn't be a big perf hit creating a new object because
this function is complicated and does things that dwarf the cost
of creating a new PyObject.