Replace backlink refs with weakrefs

- includes links to filesystem, patcher and parent dir
- in most places the filesystem is assumed (asserted) to exist

Author: mrbean-bremen

CHANGES.md

@@ -6,6 +6,8 @@ The released versions correspond to PyPI releases.
 
 ### Changes
 * added more support for PyPy 3
+* _Caution:_ many back-link references have been replaced by weak references;
+  this may have unwanted consequences (crashes) for some untested workflows
 
 ### Infrastructure
 * added PyPy 3.11 to CI, added PyPy builds for all OSes
@@ -14,6 +16,8 @@ The released versions correspond to PyPI releases.
 ### Fixes
 * fixed a problem with `readable` raising an error on a file object.
   (see [#1265](https://github.com/pytest-dev/pyfakefs/issues/1265))
+* avoid memory accumulation in consecutive tests by using weak references
+  (see [#1267](https://github.com/pytest-dev/pyfakefs/issues/1267))
 
 ## [Version 6.0.0](https://pypi.python.org/pypi/pyfakefs/6.0.0) (2025-12-21)
 Removes some deprecated functionality, removes support for Python < 3.10.