Avoid failing if __file__ doesnt exist / PyOxidizer support

[jayvdb]

PyOxidizer breaks on stdlib-list as it doesnt provide __file__ See indygreg/PyOxidizer#69 for more info about why, but the tl;dr version is __file__ is an optional attribute and should not be relied upon.

e.g. at the moment this is what it looks like

>>> import depfinder
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "depfinder", line 34, in <module>
  File "depfinder.main", line 41, in <module>
  File "stdlib_list", line 6, in <module>
  File "stdlib_list.base", line 6, in <module>
NameError: name '__file__' is not defined

[ocefpaf]

I never used PyOxidizer before. Good to know. Do you want to send a PR?