[bugfix] Set pythonpath to a fixed location relative to build path

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: easybuilders/easybuild-easyconfigs#21078

easybuild/easyblocks/p/python.py

@@ -45,7 +45,7 @@
 from easybuild.framework.easyconfig import CUSTOM
 from easybuild.framework.easyconfig.templates import TEMPLATE_CONSTANTS
 from easybuild.tools.build_log import EasyBuildError, print_warning
-from easybuild.tools.config import build_option, ERROR, log_path
+from easybuild.tools.config import build_option, ERROR
 from easybuild.tools.modules import get_software_libdir, get_software_root, get_software_version
 from easybuild.tools.filetools import apply_regex_substitutions, change_dir, mkdir
 from easybuild.tools.filetools import read_file, remove_dir, symlink, write_file
@@ -145,7 +145,7 @@ def __init__(self, *args, **kwargs):
         self.pyshortver = '.'.join(self.version.split('.')[:2])
 
         # Used for EBPYTHONPREFIXES handler script
-        self.pythonpath = os.path.join(log_path(), 'python')
+        self.pythonpath = os.path.join('easybuild', 'python')
 
         ext_defaults = {
             # Use PYPI_SOURCE as the default for source_urls of extensions.