Support downgrading newer annotations

[s0undt3ch]

Consider the following diff:

--- a/src/pytestshellutils/exceptions.py
+++ b/src/pytestshellutils/exceptions.py
@@ -3,6 +3,8 @@
 """
 Pytest Shell Utilities related exceptions.
 """
+from __future__ import annotations
+
 from typing import Optional
 
 from pytestshellutils.utils.processes import ProcessResult
@@ -30,7 +32,7 @@ class ProcessFailed(ShellUtilsException):
         The ``ProcessResult`` instance when the exception occurred
     """
 
-    def __init__(self, message: str, process_result: Optional[ProcessResult] = None) -> None:
+    def __init__(self, message: str, process_result: ProcessResult | None = None) -> None:
         super().__init__()
         self.message = message
         self.process_result = process_result

lib3to6 will fail to downgrade the code and throws the following traceback

Traceback (most recent call last):                                                                        
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/bin/downgrade-source", line 8, in <module>
    sys.exit(main())                                                                                      
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/lib/python3.8/site-packages/downgradesource/__main__.py", line 165, in main
    exitcode = untype_source(                                                                             
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/lib/python3.8/site-packages/downgradesource/__main__.py", line 57, in untype_source
    fixed_source_text = transpile.transpile_module(ctx, source_text)                                      
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/lib/python3.8/site-packages/lib3to6/transpile.py", line 290, in transpile_module
    maybe_fixed_module = fixer(ctx, module_tree)                                                          
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/lib/python3.8/site-packages/lib3to6/fixers.py", line 199, in __call__
    _FRAFContext(local_classes).remove_forward_references(tree)                                           
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/lib/python3.8/site-packages/lib3to6/fixers.py", line 186, in remove_forward_references
    self.remove_forward_references(sub_node)                                                              
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/lib/python3.8/site-packages/lib3to6/fixers.py", line 184, in remove_forward_references
    self.update_annotation_refs(sub_node, 'annotation')                                                   
  File "/home/vampas/.cache/pre-commit/repolg5j4qb6/py_env-python3.8/lib/python3.8/site-packages/lib3to6/fixers.py", line 167, in update_annotation_refs
    raise NotImplementedError(msg)                                                                                                                                                                                  
NotImplementedError: Error fixing annotation of forward ref with type <class '_ast.BinOp'>

[mbarkhau]

Certainly this shouldn't throw an error, so this is an issue on that front for sure. Earliest I can work on this is late Feb.

Whether or not it makes sense to put in the effort to do the conversion you suggested in the diff is a different question. For my use cases, to just elide the annotation is fine, but I'd accept a PR for that if somebody wants to put in the effort.