Stop requiring `__file__` in Raven, to make it work with PyOxidizer

[gjcarneiro]

See indygreg/PyOxidizer#69

[jayvdb]

The backtrace when importing raven is

  File "importlib", line 127, in import_module
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "raven", line 44, in <module>
  File "raven", line 30, in get_revision
NameError: name '__file__' is not defined

[gjcarneiro]

As someone commented in some other issue, one way to fix this issue is to switch to from raven to the new replacement sentry-sdk. I did that, and sentry-sdk works fine with PyOxidizer.