avoid modification of PYTHONPATH in flux env and the testsuite

[grondo]

After #6597 is merged, there will remain two places where Flux still, perhaps surprisingly, modifies PYTHONPATH.

One is in flux env, where PYTHONPATH continues to be modified for backwards compatibility. This is due to instructions in the Python docs here:

flux-core/doc/python/basics.rst

Lines 17 to 20 in e9f7fcd

	If you want to import the package from outside of a Flux instance,
	running ``/path/to/flux env | grep PYTHONPATH`` in your shell will show you
	what PYTHONPATH would be set to if you were running in a Flux instance
	built/installed at ``/path/to/flux``.

Perhaps a better solution would actually be to use flux config builtin python_path anyway.

Also, after #6597, sharness.d/01-setup.sh will modify PYTHONPATH so that the correct Python modules are found for any Python script run in the testsuite. This could be avoided if we instead ensure every Python script is run with flux python instead of just python3 or via a shebang line.