Add regression tests for builtins.pow and object.__reduce__ (#7663)

test_cases/README.md

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+## Regression tests for typeshed
+
+This directory contains regression tests for the stubs found elsewhere in the
+typeshed repo. Each file contains a number of test cases, all of which should
+pass a type checker without error.
+
+**This directory should *only* contain tests for functions and classes which
+are known to have caused problems in the past, where the stubs are difficult to
+get right.** 100% test coverage for typeshed is neither necessary nor
+desirable, as it would lead to code duplication. Moreover, typeshed has
+multiple other mechanisms for spotting errors in the stubs.
+
+Unlike the rest of typeshed, this directory largely contains `.py` files. This
+is because the purpose of this folder is to test the implications of typeshed
+changes for end users.
+
+Another difference to the rest of typeshed is that the test cases in this
+directory cannot always use modern syntax for type hints. For example, PEP 604
+syntax (unions with a pipe `|` operator) is new in Python 3.10. While this
+syntax can be used on older Python versions in a `.pyi` file, code using this
+syntax will fail at runtime on Python <=3.9. Since the test cases all use `.py`
+extensions, and since the tests need to pass on all Python versions >=3.6, PEP
+604 syntax cannot be used in a test case. Use `typing.Union` and
+`typing.Optional` instead.