Fix the `typing_extensions` runtime dependency version (#641)

* Only use `typing_extensions` below Python 3.8

* Rely on unconstrained `typing-extensions` version

proxy/common/types.py

@@ -10,10 +10,17 @@
 """
 import queue
 import ipaddress
+import sys
 
 from typing import TYPE_CHECKING, Dict, Any, List, Union
 
-from typing_extensions import Protocol
+# NOTE: Using try/except causes linting problems which is why it's necessary
+# NOTE: to use this mypy/pylint idiom for py36-py38 compatibility
+# Ref: https://github.com/python/typeshed/issues/3500#issuecomment-560958608
+if sys.version_info >= (3, 8):
+    from typing import Protocol
+else:
+    from typing_extensions import Protocol
 
 if TYPE_CHECKING:
     DictQueueType = queue.Queue[Dict[str, Any]]    # pragma: no cover

requirements.txt

@@ -1 +1 @@
-typing-extensions==3.10.0.2
+typing-extensions==3.10.0.2; python_version < "3.8"

setup.py

@@ -37,7 +37,9 @@ if __name__ == '__main__':
         zip_safe=False,
         packages=find_packages(exclude=['tests', 'tests.*']),
         package_data={'proxy': ['py.typed']},
-        install_requires=open('requirements.txt', 'r').read().strip().split(),
+        install_requires=[
+            'typing-extensions; python_version < "3.8"',
+        ],
         entry_points={
             'console_scripts': [
                 'proxy = proxy:entry_point'