fix: Don't hide setup_commands errors

There seems to be no problem in propagating the `stderr` outwards,
and without this there is no feedback as to what went wrong
in the `setup_commands`.

PR #258: #258

src/mkdocstrings/handlers/python.py

@@ -152,7 +152,6 @@ def __init__(self, setup_commands: Optional[List[str]] = None) -> None:
         self.process = Popen(  # noqa: S603,S607 (we trust the input, and we don't want to use the absolute path)
             cmd,
             universal_newlines=True,
-            stderr=PIPE,
             stdout=PIPE,
             stdin=PIPE,
             bufsize=-1,