ast: remove _Pattern alias workaround, use pattern directly

[hunterhogan]

Pyright no longer complains about recursive definitions for variables named pattern, so the _Pattern alias is no longer needed.

(.venv) C:\clones\typeshed>pip list
Package           Version
----------------- -------
nodeenv           1.9.1
pip               25.0.1
pyright           1.1.400
typing_extensions 4.13.2

(.venv) C:\clones\typeshed>pyright stdlib\ast.pyi --pythonversion 3.9
0 errors, 0 warnings, 0 informations 

(.venv) C:\clones\typeshed>pyright stdlib\ast.pyi --pythonversion 3.10
0 errors, 0 warnings, 0 informations 

(.venv) C:\clones\typeshed>pyright stdlib\ast.pyi --pythonversion 3.11
0 errors, 0 warnings, 0 informations 

(.venv) C:\clones\typeshed>pyright stdlib\ast.pyi --pythonversion 3.12
0 errors, 0 warnings, 0 informations 

(.venv) C:\clones\typeshed>pyright stdlib\ast.pyi --pythonversion 3.13
0 errors, 0 warnings, 0 informations 

(.venv) C:\clones\typeshed>pyright stdlib\ast.pyi --pythonversion 3.14
0 errors, 0 warnings, 0 informations 

[JelleZijlstra]

CI disagrees. If the name pattern is declared in a class scope, then usages of that name refer to the name pattern. We still need the workaround.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉