Refactor _prepare_process
Refactor _prepare_process

- simplify the way pythonpath is inserted into the process environment
- get rid of external dependency add_pathlist_to_PYTHONPATH
- follow advice at doc.qt.io/qt-5/qprocessenvironment.html#toStringList
  to not use the list obtained from toStringList with setEnvironment
- it must be `if pythonpath` instead of `if pythonpath is not None` as
  pythonpath is a list and if it's empty it's still not None so we
  would insert a variable PYTHONPATH without a value into the environment

jitseniesen/spyder-unittest