Shuffle code for starting and clearing Python in order to avoid crashes.

This addresses issue #1580 and issue #1585.

Since the _CheckIsScript execution path ends separately from the normal execution path in certain cases, it seemed easiest for now to initialize and finalize Python separately for those paths. Each of the two Python functions that can take over execution and terminate independently now initializes and finalizes Python. fontforge_main also initializes and finalizes Python after the _CheckIsScript call.