extensions: Specify dependency on typing for py<35 (#4268)

Fixes #4264

setup.py

diff --git a/setup.py b/setup.py
index 0ec4ba1..b4918f4 100644
--- a/setup.py
+++ b/setup.py
@@ -40,4 +40,7 @@
     platforms=['POSIX'],
     py_modules=['mypy_extensions'],
     classifiers=classifiers,
+    install_requires=[
+        'typing >= 3.5.3; python_version < "3.5"',
+    ],
 )