don't attempt to "correct" PYTHONPATH configuration for subprocesses unless asked

[glyph]

When Ampoule was first written, it was common practice for applications to do weird, potentially non-replicable startup nonsense to get sys.path to be correct and include all of one's sources. Since virtualenv, venv, and pip have taken over the Python developer-setup space, it's much more likely that your Python's path and dependency configuration is correct if you just don't mess with it.

Setting PYTHONPATH to point at the standard dist dirs can also have weird and unintended side-effects. For example, any shadowing of stdlib modules (enum34, for example) might create inscrutable breakages when .pth files try to import things (like coverage).

#24 is another symptom of this monkeying around with sys.path.

Let's leave the environment untouched, and provide a hook to tweak it in the now-unusual case where something has broken.

[graingert]

@glyph there's already a hook - the user can pass env={"PYTHONPATH": ...}

[glyph]

@graingert fair enough! that does seem sufficient.