Add def multiprocessing_breakpoint function

Using the builtin `breakpoint` function does not work with
multiprocessing, as the Process forks has different STDIN so the pdb can
not attach to it. Use this `multiprocessing_breakpoint` function
instead, when you need a breakpoint for debugging.

unblob/logging.py

@@ -1,4 +1,6 @@
 import logging
+import pdb
+import sys
 from os import getpid
 from pathlib import Path
 from typing import Any
@@ -79,3 +81,18 @@ def configure_logger(verbose: bool, extract_root: Path):
         wrapper_class=structlog.make_filtering_bound_logger(log_level),
         processors=processors,
     )
+
+
+class _MultiprocessingPdb(pdb.Pdb):
+    def interaction(self, *args, **kwargs):
+        _stdin = sys.stdin
+        try:
+            sys.stdin = open("/dev/stdin")
+            pdb.Pdb.interaction(self, *args, **kwargs)
+        finally:
+            sys.stdin = _stdin
+
+
+def multiprocessing_breakpoint():
+    """Call this in Process forks instead of the builtin `breakpoint` function for debugging with PDB."""
+    return _MultiprocessingPdb().set_trace()