You could use the kernel `isolcpus` option in conjunction with the `taskset` command.
On the Raspberry Pi reserve the core(s) you want to use by appending the following to the line in `/boot/cmdline.txt`.
E.g. to reserve cores 2 and 3.
isolcpus=2,3
Then use taskset to assign programs to the core(s).
E.g. to launch the Python interpreter.
taskset -c 3 python
(look here for information on how to use the command).
If this is Raspberry Pi specific do you need to bit bang? There may be already existing solutions.