Install websockets version dependent on python version

Author: kclowes

setup.py

@@ -90,7 +90,10 @@
         "requests>=2.16.0,<3.0.0",
         # remove typing_extensions after python_requires>=3.8, see web3._utils.compat
         "typing-extensions>=3.7.4.1,<5;python_version<'3.8'",
-        "websockets>=9.1,<11",
+        # websockets 10.0 removed support for python 3.6.
+        # We can remove this line once we drop 3.6 support as well
+        "websockets>=9.1,<10;python_version<'3.7'",
+        "websockets>=9.1,<11;python_version>'3.6'",
     ],
     python_requires='>=3.6,<4',
     extras_require=extras_require,