Fix errors with multiprocessing

Before, one could get OSError 22 and BadZipFile errors due to re-used file pointers in forked subprocesses.

Fixes #520

Author: 2xB

importlib_metadata/__init__.py

@@ -802,6 +802,7 @@ class FastPath:
     True
     """
 
+    # The following cache is cleared at fork, see os.register_at_fork below
     @functools.lru_cache()  # type: ignore[misc]
     def __new__(cls, root):
         return super().__new__(cls)
@@ -842,6 +843,10 @@ def mtime(self):
     def lookup(self, mtime):
         return Lookup(self)
 
+# Clear FastPath.__new__ cache when forked, avoids trying to re-useing open 
+# file pointers from zipp.Path/zipfile.Path objects in forked process
+os.register_at_fork(after_in_child=FastPath.__new__.cache_clear)
+
 
 class Lookup:
     """