Simplify typing_extensions pin

We fixed the PyPy issues in [typing_extensions 4.7.1](https://github.com/python/typing_extensions/releases/tag/4.7.1) by working around [the bug in PyPy-3.7 and PyPy-3.8](https://foss.heptapod.net/pypy/pypy/-/issues/3958).

Loosening the pin here will give us the option of running pydantic's tests with typing_extensions's main branch on more PyPy versions in CI as part of our [third-party nightly workflow](https://github.com/python/typing_extensions/blob/main/.github/workflows/third_party.yml). Doing that should hopefully reduce the chance of breakages like this in the future :)

Author: AlexWaygood

pyproject.toml

@@ -1,8 +1,7 @@
 [build-system]
 requires = [
     'maturin>=1,<2',
-    'typing-extensions >=4.6.0; platform_python_implementation != "PyPy" or python_version >= "3.9"',
-    'typing-extensions >=4.6.0,<4.7.0; platform_python_implementation == "PyPy" and python_version < "3.9"'
+    'typing-extensions >=4.6.0,!=4.7.0'
 ]
 build-backend = 'maturin'
 
@@ -33,8 +32,7 @@ classifiers = [
     'Typing :: Typed',
 ]
 dependencies = [
-    'typing-extensions >=4.6.0; platform_python_implementation != "PyPy" or python_version >= "3.9"',
-    'typing-extensions >=4.6.0,<4.7.0; platform_python_implementation == "PyPy" and python_version < "3.9"'
+    'typing-extensions >=4.6.0,!=4.7.0'
 ]
 dynamic = [
     'description',