PYTHONPATH is cleared by default rather than preserved

[heyakyra]

The code in question: https://github.com/twisted/ampoule/blob/master/ampoule/main.py#L268

This makes some modules impossible to find

[glyph]

Hi @heyakyra - thanks for reporting this issue. A couple of years later this indeed bit me in production, so I wish I'd responded to the issue at the time :)