change custom easyblock for Python to set `pythonpath` class variable to a fixed location relative to build path

[gkaf89]

The pythonpath variable in the python.py EasyBlock is used to create temporary directories to store build artifacts. However, there are 2 problems with the current definition of the pythonpath instance field:

• it uses the log directory to store build artifacts, and
• the functions that use pythonpath do not accept a full path.

The build_path() function cannot be used instead of log_path() as the function provides a full path as well. Thus we opted to hard-code a relative path with respect to the build directory.

Issue: https://github.com/easybuilders/easybuild-easyconfigs/issues/21078