allow typing-extensions-4.7.0+ on pypy3.9+

Remove the PyPy-specific version restriction on typing-extensions when
PyPy3.9 or newer is used.  The bug cited
in 12073b8 is specific to PyPy3.7
and PyPy3.8.

Author: mgorny

pyproject.toml

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