Fix installation with pypy3.7 and pypy3.8 (#716)

Co-authored-by: David Montague <[email protected]>

Author: adriangb

pyproject.toml

@@ -1,5 +1,9 @@
 [build-system]
-requires = ['maturin>=1,<2', 'typing_extensions']
+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"'
+]
 build-backend = 'maturin'
 
 [project]
@@ -29,7 +33,8 @@ classifiers = [
     'Typing :: Typed',
 ]
 dependencies = [
-    'typing_extensions; python_version < "3.11.0"'
+    'typing-extensions >=4.6.0; platform_python_implementation != "PyPy"',
+    'typing-extensions >=4.6.0,<4.7.0; platform_python_implementation == "PyPy"'
 ]
 dynamic = [
     'description',